Gang leader? Very nearly! (6)
I believe the answer is:
almost
'nearly' is the definition.
(synonyms)
'gang leader? very' is the wordplay.
'gang leader?' becomes 'al' (Al Capone).
'very' becomes 'most' (I've seen this in another clue).
'al'+'most'='ALMOST'
(Other definitions for almost that I've seen before include "As good as" , "Well-nigh" , "practically" , "Not totally" , "Nearly, not quite" .)