Thirty seconds, perhaps, to put on one’s skirt (4)
I believe the answer is:
mini
'skirt' is the definition.
(a mini skirt)
'thirty seconds perhaps to put on one's' is the wordplay.
'thirty seconds perhaps' becomes 'min' (short for minute).
'to put on' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'min'+'i'='MINI'
(Other definitions for mini that I've seen before include "Small (of car, skirt, disk,bar or series)" , "Brief" , "Italian Job car?" , "Car; short dress" , "Little - car" .)