Harmless during attack (11)
I believe the answer is:
inoffensive
'harmless' is the definition.
(similar in meaning)
'during attack' is the wordplay.
I cannot quite understand how this works, but
'during' could be 'in' (e.g. during February means in February) and 'in' is found in the answer.
'attack' could be 'offensive' (similar in meaning) and 'offensive' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for inoffensive that I've seen is " Harmless -- not objectionable".)