A number well-suited to fashion (5)
I believe the answer is:
adapt
'to fashion' is the definition.
(I've seen this before)
'a number well-suited' is the wordplay.
'number' becomes 'D' (D is an example).
'well suited' becomes 'apt' (I've seen this in another clue).
'a'+'d'+'apt'='ADAPT'
(Other definitions for adapt that I've seen before include "Become adjusted to new conditions" , "match" , "get used?" , "Fashion" , "Change to suit new circumstances" .)