Dull colour of old poet, ill, withdrawn (5,4)
I believe the answer is:
olive drab
'dull colour' is the definition.
(I've seen this before)
'old poet ill withdrawn' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'poet' becomes 'bard' (bard is a kind of poet).
'ill' becomes 'evil'.
'withdrawn' says the letters should be written backwards.
'bard'+'evil'='bardevil'
'bardevil' reversed gives 'livedrab'.
'o'+'livedrab'='OLIVE-DRAB'
'of' is the link.
(Other definitions for olive drab that I've seen before include "Military uniform colour" , "a shade unsparkling, maybe" , "Greyish" , "Dull colour" , "Mil. uniform colour" .)