... to maintain shell protecting core of machinery (5)
I believe the answer is:
claim
'to maintain' is the definition.
(thesaurus)
'shell protecting core of machinery' is the wordplay.
'shell' becomes 'clam' (I am not sure about this - if you are sure you should believe this answer much more).
'protecting' is an insertion indicator.
'core of' indicates the central letters.
The middle of 'machinery' is 'i'.
'clam' placed around 'i' is 'CLAIM'.
Can you help me to learn more?
(Other definitions for claim that I've seen before include "Right; demand this" , "Call" , "Assertion; demand" , "Assert one's right to" , "Assertion of ownership etc" .)