Ask earnestly to be good (3)
I believe the answer is:
beg
'ask earnestly' is the definition.
(I've seen this in another clue)
'be good' is the wordplay.
'good' becomes 'g' (abbreviation).
'be'+'g'='BEG'
'to' acts as a link.
(Other definitions for beg that I've seen before include "Plead or importune" , "Earnestly entreat" , "Plead, implore" , "Ask humbly" , "Solicit charity" .)