Note stink over ball delivery (7)
I believe the answer is:
observe
'note' is the definition.
('observe' can be a synonym of 'note')
'stink over ball delivery' is the wordplay.
'stink' becomes 'bo' (body odour).
'over' shows that the letters should be reversed in order.
'ball delivery' becomes 'serve' (delivery of the ball in tennis).
'bo' reversed gives 'ob'.
'ob'+'serve'='OBSERVE'
(Other definitions for observe that I've seen before include "Notice or conform to, rules say" , "Notice; study" , "Keep an eye on" , "Look at" , "Scrutinise or comply with" .)