Attempts to get iron back to castles (7)
I believe the answer is:
efforts
'attempts' is the definition.
('effort' can be a synonym of 'attempt')
'get iron back to castles' is the wordplay.
'get' is a charade indicator (letters next to each other).
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'back' says the letters should be written backwards.
'to castles' becomes 'forts' (I've seen this before).
'fe' reversed gives 'ef'.
'ef' next to 'forts' is 'EFFORTS'.
'to' is the link.
(Other definitions for efforts that I've seen before include "Determined attempts" , "Acts of trying hard" , "Endeavours, attempts" , "Exertions" .)