Check cover on trailer after parking (9)
I believe the answer is:
proofread
'check' is the definition.
(I know that proofread is a more specific form of the action check)
'cover on trailer after parking' is the wordplay.
'cover' becomes 'roof' (roofing is a kind of covering).
'on' becomes 're' ('on' can be mean 'with reference to').
'trailer' becomes 'ad' (short for advert).
'after' says to put letters next to each other.
'parking' becomes 'p' (abbreviation on road signs).
'roof'+'re'+'ad'='roofread'
'roofread' put after 'p' is 'PROOFREAD'.
(Other definitions for proofread that I've seen before include "correct text" , "Prepare for publication" , "Carefully check" , "Checked text" .)