Grouch taking run in taxi (4)
I believe the answer is:
crab
'grouch' is the definition.
(I know that crab is a type of grouch)
'run in taxi' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'in' is an insertion indicator.
'taxi' becomes 'cab' (synonyms).
'r' put within 'cab' is 'CRAB'.
'taking' acts as a link.
(Other definitions for crab that I've seen before include "The constellation Cancer" , "Shellfish with pincers" , "Scuttling crustacean" , "Fiddler, perhaps" , "Seashore crustacean" .)