Instruction to leave second offensive (4)
I believe the answer is:
stet
'instruction' is the definition.
(I have seen 'marginal instruction' mean 'stet' so perhaps 'instruction' could also mean 'stet')
'leave second offensive' is the wordplay.
'leave' indicates taking the first letters (I've seen 'left of' mean this (the leftmost letter in the word)).
'offensive' becomes 'tet' (Tet Offensive, Vietnam War military campaign).
The initial letter of 'second' is 's'.
's'+'tet'='STET'
'to' acts as a link.
(Other definitions for stet that I've seen before include "'Let it stand, in printing (4)'" , "Leave as is - don't amend" , "Notation meaning leave as is, don't alter spelling" , "Ignore crossing" , "Notation for let spelling stand, don't change it" .)