The fish will be rising right away (5)
I believe the answer is:
trout
'the fish' is the definition.
(trout is a kind of fish)
'will be rising right away' is the wordplay.
'will' means one lot of letters go next to another (I've seen this in other clues).
'be rising' says the letters should be written backwards (in a down clue, letters go up).
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'away' becomes 'out' (similar in meaning).
'rt' back-to-front is 'tr'.
'tr' next to 'out' is 'TROUT'.
(Other definitions for trout that I've seen before include "Mistiness" , "Unattractive old woman" , "air" , "Oily fish" , "grumpy old woman" .)