Crustacean runs inside taxi (4)
I believe the answer is:
crab
'crustacean' is the definition.
(I know that crab is a type of decapod)
'runs inside taxi' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'inside' indicates putting letters inside.
'taxi' becomes 'cab' (synonyms).
'r' put into 'cab' is 'CRAB'.
(Other definitions for crab that I've seen before include "Marine crustacean (seen on 1 across?)" , "Nautical nipper" , "Clawed crustacean" , "Seafood (that can be caught by a rower?)" , "Clawed shellfish" .)