Yell cry of pain, being immature (6)
I believe the answer is:
callow
'immature' is the definition.
('callow' can be similar in meaning to 'immature')
'yell cry of pain' is the wordplay.
'yell' becomes 'call' ('call' can be a synonym of 'yell').
'cry of pain' becomes 'ow' (I've seen this in other clues).
'call'+'ow'='CALLOW'
'being' is the link.
(Other definitions for callow that I've seen before include "Inexperienced or innocent" , "Immature and inexperienced, like youth" , "Green" , "23 [BRIGHTONIAN] actor" , "Immature and inexperienced, as youth often is" .)