A new gang-leader, Heather, is fishing (7)
I believe the answer is:
angling
'is fishing' is the definition.
(I've seen this in another clue)
'a new gang-leader heather' is the wordplay.
'leader' suggests taking the first letters.
'heather' becomes 'ling' (synonyms).
The initial letters of 'new gang' is 'ng'.
'a'+'ng'+'ling'='ANGLING'
(Other definitions for angling that I've seen before include "Pursuit with hook, line and sinker?" , "Pointing" , "Fishing with rod and line" , "seeking compliments, perhaps?" , "Fishing with hook and line" .)