Resolve mainly intact, yet must be broken (8)
I believe the answer is:
tenacity
'resolve' is the definition.
(tenacity is a kind of resolve)
'mainly intact yet must be broken' is the wordplay.
'mainly' means to remove the last letter.
'must be broken' is an anagram indicator.
'intact'+'yet'='intactyet'
'intactyet' with its last letter removed is 'intactye'.
'intactye' is an anagram of 'TENACITY'.
(Other definitions for tenacity that I've seen before include "Firmness of hold" , "Dogged resolve" , "Firm grasp" , "Persistent determination, doggedness" , "Perseverance, doggedness" .)