Medical hell (to put it mildly) stuck in a metal casing (7)
I believe the answer is:
checkup
'medical' is the definition.
(synonyms)
'hell to put it mildly stuck in a metal casing' is the wordplay.
I cannot really see how this works, but
'hell' could be 'heck' (interjections!) and 'heck' is located in the answer.
'metal' could be 'u' (U is an example) and 'u' is found in 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 checkup that I've seen before include "Quick dental visit" , "Precautionary or routine visit to G.P." , "An occasional precautionary medical examination" .)