Pull back, adopting new custom (4)
I believe the answer is:
wont
'custom' is the definition.
(wont is a kind of custom)
'pull back adopting new' is the wordplay.
'pull' becomes 'tow' (tow is a kind of pull).
'back' shows that the letters should be reversed in order.
'adopting' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'tow' in reverse letter order is 'wot'.
'wot' going around 'n' is 'WONT'.
(Other definitions for wont that I've seen before include "Succinct refusal" , "normal practice" , "customary behaviour" , "Habit" , "Accustomed" .)