Will start to provide testimonial (10)
I believe the answer is:
preference
'will' is the definition.
(thesaurus)
'start to provide testimonial' is the wordplay.
'start to' suggests taking the first letters.
'testimonial' becomes 'reference' (reference is a kind of testimonial).
The initial letter of 'provide' is 'p'.
'p'+'reference'='PREFERENCE'
(Other definitions for preference that I've seen before include "Predilection, penchant" , "Choice (of A rather than B)" , "what's valued higher" , "Liking for one thing more than another" , "favourite" .)