Dull quality of poet backed by head (8)
I believe the answer is:
drabness
'dull quality' is the definition.
(being drab or dull)
'poet backed by head' is the wordplay.
'poet' becomes 'bard' (bard is a kind of poet).
'backed' is a reversal indicator.
'by' means one lot of letters go next to another.
'head' becomes 'ness' (ness is a headland).
'bard' reversed gives 'drab'.
'drab'+'ness'='DRABNESS'
'of' acts as a link.
(Other definitions for drabness that I've seen before include "Dowdiness, dreariness" , "Dull quality" .)