Walk unsteadily, initially tumbling over fish-eating mammal (6)
I believe the answer is:
totter
'walk unsteadily' is the definition.
(I've seen this before)
'initially tumbling over fish-eating mammal' is the wordplay.
'initially' suggests taking the first letters.
'over' means one lot of letters go next to another.
'fish-eating mammal' becomes 'otter' (I've seen this before).
The first letter of 'tumbling' is 't'.
't'+'otter'='TOTTER'
(Other definitions for totter that I've seen before include "Lurch" , "Stand/walk unsteadily" , "Rag-and-bone man - summer" , "Scrap dealer" , "Move unsteadily; be about to fall" .)