Representative sample of collagen treatment (5)
I believe the answer is:
agent
'representative' is the definition.
(agent is a kind of representative)
'sample of collagen treatment' is the wordplay.
'sample of' indicates the answer is hidden within the clue (an extract or sample of the clue).
'AGENT' is hidden within 'collagen treatment'.
(Other definitions for agent that I've seen before include "Traveller" , "Sales rep" , "Substance that exerts some force or effect" , "Actors' manager" , "Performer's manager" .)