Lines of attack rejected (5)
I believe the answer is:
notes
'lines' is the definition.
('note' can be a synonym of 'line')
'of attack rejected' is the wordplay.
'of' becomes 'no' (I am not sure about this - if you are sure you should believe this answer much more).
'attack' becomes 'set' (setting is a kind of attacking).
'rejected' shows that the letters should be reversed in order.
'set' in reverse letter order is 'tes'.
'no'+'tes'='NOTES'
Can you help me to learn more?
(Other definitions for notes that I've seen before include "Memos; marks" , "Sounds" , "fivers?" , "Observes; short comments" , "eg E F G (mus.)" .)