I gasp following quiet lorry — one involved in trial (11)
I believe the answer is:
participant
'one involved in trial' is the definition.
(participant in a trial)
'i gasp following quiet lorry' is the wordplay.
'gasp' becomes 'pant' (synonyms).
'following' says to put letters next to each other.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'lorry' becomes 'artic' (informal term for an articulated lorry).
'i'+'pant'='ipant'
'p'+'artic'='partic'
'ipant' after 'partic' is 'PARTICIPANT'.
(Other definitions for participant that I've seen before include "One getting involved in something" , "Actor" , "Competitor" , "One actively involved" , "player" .)