Force put into iron structure (5)
I believe the answer is:
frame
'structure' is the definition.
(I know that frame is a type of body of vertebrate)
'force put into iron' is the wordplay.
'force' becomes 'ram' (synonyms).
'put into' is an insertion indicator.
'iron' becomes 'fe' (chemical symbol for iron).
'ram' put into 'fe' is 'FRAME'.
(Other definitions for frame that I've seen before include "Supporting, enclosing, structure" , "Enclosing case or border" , "Underlying structure" , "Picture holder" , "Body; shape" .)