Unexpected gains in turn decline for spells (8,7)

'unexpected gains' is the definition.
(windfall profits are unexpected profits)

'turn decline for spells' is the wordplay.
I cannot quite see how this works, but
'turn' could be 'wind' (I've seen this before) and 'wind' is located in the answer.
'decline' could be 'fall' (synonyms) and 'fall' is found in the answer.
an anagram of 'for' is 'rof' which is located in the answer.
The remaining letters 'pits' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'in' is the link.