Decisive tip from fisherman in a lake (5)
I believe the answer is:
final
'decisive' is the definition.
(I've seen this in another clue)
'tip from fisherman in a lake' is the wordplay.
'tip from' indicates taking the first letters.
'lake' becomes 'L' (geographical abbreviation).
The initial letter of 'fisherman' is 'f'.
'f'+'in'+'a'+'l'='FINAL'
(Other definitions for final that I've seen before include "Last, ultimate" , "Conclusive" , "Like one's journey to cemetery" , "big match" , "Last in a series of games or contests" .)