Unsteady river animal in banks of Tay (7)
I believe the answer is:
tottery
'unsteady' is the definition.
(similar in meaning)
'river animal in banks of tay' is the wordplay.
'river animal' becomes 'otter' (animal that may live by a river).
'in' means one lot of letters goes inside another.
'banks of' suggests removing the centre (the banks are the sides of the word).
'tay' with its middle removed is 'ty'.
'otter' going into 'ty' is 'TOTTERY'.
(Other definitions for tottery that I've seen before include "Moving unsteadily" , "close to collapse" , "Moving insecurely" , "Unsteady (on one's feet)" , "Frail" .)