Climbed like a fish (6)
I believe the answer is:
scaled
I believe this clue is a double definition.
'climbed' is the first definition.
(scaling is a kind of climbing)
'like a fish?' is the second definition.
(I've seen this before)
(Other definitions for scaled that I've seen before include "like a snake" , "Climbed; flaked off" , "(Of a body) covered with thin horny plates" , "Climbed, or removed fish flakes" , "Weighed" .)