Pivotal point of fishing expedition (5)
I believe the answer is:
hinge
'pivotal point' is the definition.
(I've seen this before)
'of fishing expedition' is the wordplay.
'of' indicates a hidden word.
'HINGE' is hidden within 'fishing expedition'.
(Other definitions for hinge that I've seen before include "Joint on which things turn" , "Joint between door and frame" , "Fastening for door etc" , "Door pivot" , "Flexible joint" .)