Mum purchasing cool, small ... (6)
I believe the answer is:
minute
'small' is the definition.
(I've seen this before)
'mum purchasing cool' is the wordplay.
'mum' becomes 'mute' (to keep mum is to remain silent).
'purchasing' indicates putting letters inside (some letters take possession of others).
'cool' becomes 'in' (I've seen this in other clues).
'mute' placed around 'in' is 'MINUTE'.
(Other definitions for minute that I've seen before include "Short period of time" , "Little - bit of time" , "Tiny; written note" , "60 seconds; tiny" , "not much time" .)