S. R. Faruqi:

The safedī kā sāyah (that is, a delicate suspicion of the smallest hint of whitewash)-- how superb it is! Firaq Sahib had written, in his own praise, that in his poetry was found the 'union of opposites' [ittiḥād-e ẓiddain]. In Firaq Sahib's poetry I didn't find any 'union of opposites', but in this verse of Mir's I certainly see a living example.

By pare is meant 'ranks of troops'. When the night of separation works its cruelty-- that is, when it spreads its blackness-- it advances to such an extent that when it sees even hints or shadows of whitewash then it attacks forcefully, as though there would be an assault by rank upon rank of troops.

Then, sar karnā can mean 'to conquer', and can also mean 'to establish, to begin'. See {757,3}.

FWP:

SETS == OPPOSITES
MOTIFS
NAMES
TERMS

For discussion of textual problems in this ghazal, see {757,1}. I have used the kulliyat text rather than the one given in SSA. SRF however is using the SSA version of the first line: yih shab-e hijr sar kare nah rahe .