Fashion's to purchase small mitts (5)
I believe the answer is:
fists
'mitts' is the definition.
(fist is a kind of mitt)
'fashion's to purchase small' is the wordplay.
'fashion's' becomes 'fits' (fit is a kind of fashion).
'to purchase' means one lot of letters goes inside another (inserted letters are purchased or acquired).
'small' becomes 's' (abbreviation - e.g. clothes size).
'fits' placed around 's' is 'FISTS'.
(Other definitions for fists that I've seen before include "Fighting tools" , "Doubled up hands" , "bunches of five" , "addition to arms?" , "Hands with fingers clenched" .)