Cheat to get a crab (7)
I believe the answer is:
scamper
'crab' is the definition.
(I know that crab is a more specific form of the action scamper)
'cheat to get a' is the wordplay.
'cheat' becomes 'scam' (scam is a kind of cheat).
'to get' is a charade indicator (letters next to each other).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'scam'+'per'='SCAMPER'
(Other definitions for scamper that I've seen before include "Proceed hurriedly" , "Skip playfully with campers" , "Run and skip playfully about" , "Scurry, scuttle" , "Beetle" .)