Lean on musical instrument, keeling over (4)
I believe the answer is:
abut
'lean on' is the definition.
(I've seen this before)
'musical instrument keeling over' is the wordplay.
'musical instrument' becomes 'tuba' (tuba is a kind of musical instrument).
'keeling over' is a reversal indicator.
'tuba' in reverse letter order is 'ABUT'.
(Other definitions for abut that I've seen before include "Cease (from activity)" , "Lie adjacent to" , "Be next to -- share a boundary" , "Touch (side of next building)" , "Adjoin, touch" .)