Were swallowed by a fish, so shrank! (7)
I believe the answer is:
cowered
'shrank' is the definition.
(I've seen this in another clue)
'were swallowed by a fish' is the wordplay.
'swallowed by' means one lot of letters goes inside another.
'a fish' becomes 'cod' (type of fish).
'were' going inside 'cod' is 'COWERED'.
'so' is the link.
(Other definitions for cowered that I've seen before include "Showed fear" , "looked fearful" , "Cringed" .)