What's more it's a sincere conversion (8)
I believe the answer is:
increase
'what's more' is the definition.
(I've seen this before)
'a sincere conversion' is the wordplay.
'conversion' is an anagram indicator.
'a'+'sincere'='asincere'
'asincere' with letters rearranged gives 'INCREASE'.
'it's' is the link.
(Other definitions for increase that I've seen before include "Grow greater" , "extra money at work" , "What's more" , "Amplification" , "Breed - nicer sea (anag)" .)