Poor work between the lines (4)
I believe the answer is:
ropy
'poor' is the definition.
(thesaurus)
'work between the lines' is the wordplay.
'work' becomes 'op' (operation, work done in an operating theatre).
'between' indicates putting letters inside.
'the lines' becomes 'ry' (abbreviation for railway).
'op' placed into 'ry' is 'ROPY'.
(Other definitions for ropy that I've seen before include "Far from top quality" , "Of dubious quality" , "Slightly unwell" , "Of poor quality (informal)" , "Not very good" .)