Rusty old, old nails very, very strong, marines concluding (3,4)
I believe the answer is:
off form
'concluding' is the definition.
I know nothing about this answer so I can't judge whether this works.
'rusty old old nails very very strong marines' is the wordplay.
I cannot quite see how this works, but
'old' could be 'o' (abbreviation) and 'o' is found within the answer.
'strong' could be 'ff' and 'ff' is present in the answer.
'marines' could be 'rm' (abbreviation for 'Royal Marines') and 'rm' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for off form that I've seen before include "playing badly" , "Describing indifferent quality" , "Not at one's best" .)