Fish rising means flexibility (6)
I believe the answer is:
leeway
'flexibility' is the definition.
(I've seen this before)
'fish rising means' is the wordplay.
'fish' becomes 'eel' (eel is a kind of fish**).
'rising' shows that the letters should be reversed in order (going up in a down clue).
'means' becomes 'way' ('way' can be a synonym of 'means').
'eel' written backwards gives 'lee'.
'lee'+'way'='LEEWAY'
(Other definitions for leeway that I've seen before include "Limited room for manoeuvre" , "Elbowroom" , "Room for movement within limits" , "Allowance" , "Amount of freedom" .)