Poet with horrible old retro GI uniform (5,4)
I believe the answer is:
olive drab
'gi uniform' is the definition.
(colour of an American GI's uniform)
'poet with horrible old retro' is the wordplay.
'poet' becomes 'bard' (bard is a kind of poet).
'with horrible' becomes 'evil' (I can't explain this - if you can you should believe this answer much more).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'retro' shows that the letters should be reversed in order (retro is Latin for 'backwards').
'bard'+'evil'+'o'='bardevilo'
'bardevilo' in reverse letter order is 'OLIVE-DRAB'.
(Other definitions for olive drab that I've seen before include "Colour of some military uniforms" , "Mil. uniform colour" , "Soldiers' uniform" , "Dull colour" , "Greyish" .)