Tuna scarce? Switch to crab, perhaps (10)
I believe the answer is:
crustacean
'crab perhaps' is the definition.
(crab is a kind of crustacean)
'tuna scarce? switch' is the wordplay.
'switch' indicates an anagram.
'tuna'+'scarce'='tunascarce'
'tunascarce' is an anagram of 'CRUSTACEAN'.
'to' is the link.
(Other definitions for crustacean that I've seen before include "seafood" , "crab maybe" , "Perhaps crab" , "Such as a crab or shrimp" , "Such as a barnacle" .)