Poet joins volunteers providing cover for knight (6)
I believe the answer is:
tabard
'cover for knight' is the definition.
(garment worn by a knight)
'poet joins volunteers' is the wordplay.
'poet' becomes 'bard' (bard is a kind of poet).
'joins' is a charade indicator (letters next to each other).
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'bard' after 'ta' is 'TABARD'.
'providing' acts as a link.
(Other definitions for tabard that I've seen before include "[A] top" , "Herald's garment" , "Short sleeveless tunic" , "Sleeveless jacket" , "Inn where pilgrims met" .)