Horse to move unsteadily round run (7)
I believe the answer is:
trotter
'horse' is the definition.
(trotter is a kind of horse)
'move unsteadily round run' is the wordplay.
'move unsteadily' becomes 'totter' (I've seen this before).
'round' is an insertion indicator.
'run' becomes 'r' (cricket abbreviation).
'totter' placed around 'r' is 'TROTTER'.
'to' is the link.
(Other definitions for trotter that I've seen before include "Pig's foot, or one on the run" , "Briskly moving horse - pig's foot" , "A slow runner or a pig's foot" , "bit of a pig" , "Foot of pig" .)