Control on inside (4)
I believe the answer is:
rein
'control' is the definition.
(rein is a kind of control)
'on inside' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'inside' becomes 'in'.
're'+'in'='REIN'
(Other definitions for rein that I've seen before include "Controlling strap" , "kidney?" , "Slow-up" , "Strap to control horse" , "A means of control" .)