Runner's trouble with green pants (9)
I believe the answer is:
messenger
'runner's' is the definition.
(I've seen this before)
'trouble with green pants' is the wordplay.
'trouble' becomes 'mess' ().
'with' says to put letters next to each other.
'pants' indicates an anagram (UK informal term for 'nonsense' or 'rubbish').
'green' anagrammed gives 'enger'.
'mess'+'enger'='MESSENGER'
(Other definitions for messenger that I've seen before include "Bearer of communication" , "Delivery boy" , "Kind of bearer - don't shoot him" , "Runner" , "Courier - don't shoot him" .)