Haunt deserted library many a time (10)
I believe the answer is:
'many a time' is the definition.
'haunt deserted library' is the wordplay.
'haunt' becomes 'frequent' (I can't explain this - if you can you should believe this answer much more).
'deserted' says to hollow out the word (remove centre letters) (middle letters must vacate or desert the word).
'library' with its centre taken out is 'ly'.
Can you help me to learn more?
(Another definition for frequently that I've seen is " Often".)