Terrier frolicking in car (5)
I believe the answer is:
cairn
'terrier' is the definition.
(I know that cairn is a type of terrier)
'frolicking in car' is the wordplay.
'frolicking' is an anagram indicator.
'in'+'car'='incar'
'incar' with letters rearranged gives 'CAIRN'.
(Other definitions for cairn that I've seen before include "Stone-pile landmark" , "Man" , "Terrier - mound of stones" , "Rocky pile" , "Stones piled up as a marker" .)