Trolley carrying exercise mat (6)
I believe the answer is:
carpet
'mat' is the definition.
(thesaurus)
'trolley carrying exercise' is the wordplay.
'trolley' becomes 'cart' (I've seen this in another clue).
'carrying' means one lot of letters goes inside another.
'exercise' becomes 'pe' (Physical Education).
'cart' going around 'pe' is 'CARPET'.
(Other definitions for carpet that I've seen before include "What may deaden sound in room" , "blanket" , "Dress down" , "Tear into - floor covering" , "Implied but not stated" .)