Lean on inverted instrument (4)
I believe the answer is:
tuba
'instrument' is the definition.
(tuba is a kind of instrument)
'lean on inverted' is the wordplay.
'lean on' becomes 'abut' (I've seen this before).
'inverted' shows that the letters should be reversed in order.
'abut' written backwards gives 'TUBA'.
(Other definitions for tuba that I've seen before include "member of band" , "This loud instrument" , "One's blown" , "Large low-pitched brass musical instrument" , "A bass instrument" .)