Beneath part of main frame (5)
I believe the answer is:
infra
'beneath part' is the definition.
Both the answer and definition are adverbs. Perhaps you can see an association between them that I don't see?
'of main frame' is the wordplay.
'of' indicates a hidden word.
'INFRA' is hidden within 'main frame'.
Can you help me to learn more?
(Other definitions for infra that I've seen before include "Latin adverb meaning below or further on in book" , "Coming below" , "Below (as prefix)" , "Beneath (esp. 15)" , "lower down" .)