Forbidding old, less inspiring love (7)
I believe the answer is:
ominous
'forbidding' is the definition.
('ominous' can be a synonym of 'forbidding')
'old less inspiring love' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'less' becomes 'minus' (I've seen this before).
'inspiring' is an insertion indicator.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'minus' going around 'o' is 'minous'.
'o'+'minous'='OMINOUS'
(Other definitions for ominous that I've seen before include "'Threatening, foreshadowing tragedy (7)'" , "Threatening, foreshadowing something bad" , "Looking like trouble coming" , "Portending bad things" , "Threatening, doesn't look good" .)