Take in a smile as it develops (10)
I believe the answer is:
assimilate
'take in' is the definition.
('assimilate' can be a synonym of 'take in')
'a smile as it develops' is the wordplay.
'develops' indicates anagramming the letters.
'smile'+'as'+'it'='smileasit'
'smileasit' anagrammed gives 'ssimilate'.
'a'+'ssimilate'='ASSIMILATE'
(Other definitions for assimilate that I've seen before include "Become part of the prevailing culture" , "Take in and understand" , "pick up" , "Absorb and digest (food)" , "thoroughly understood" .)