Promptly forward after that trouble (2,3,4)
I believe the answer is:
on the nail
'promptly' is the definition.
(I've seen this before)
'forward after that trouble' is the wordplay.
'forward' becomes 'on'.
'after' becomes 'then'.
'that trouble' becomes 'ail' (I've seen this before).
'on'+'then'+'ail'='ON THE NAIL'
(Other definitions for on the nail that I've seen before include "Where a coat may be hung" , "location of solution for digital protection" , "immediately" , "without delay" , "Prompt" .)