Chatter and endlessly mull over love -- what a pain! (7)
I believe the answer is:
lumbago
'what a pain' is the definition.
(lumbago is a kind of pain)
'chatter and endlessly mull over love' is the wordplay.
'chatter' becomes 'gab' (I've seen this before).
'and' is a charade indicator (letters next to each other).
'endlessly' means to remove the last letter.
'over' says the letters should be written backwards.
'love' becomes 'o' (In tennis, 'love' means 'zero'**).
'mull' with its last letter removed is 'mul'.
'gab'+'mul'='gabmul'
'gabmul' backwards is 'lumbag'.
'lumbag'+'o'='LUMBAGO'
(Other definitions for lumbago that I've seen before include "Maul bog badly for a rheumatic affliction" , "Rheumatic pain in the small of the back" , "Back problem" , "Back complaint" , "Rheumatism of lower back" .)