Annoying very old partner, one no good (6)
I believe the answer is:
vexing
'annoying' is the definition.
('vex' can be a synonym of 'annoy')
'very old partner one no good' is the wordplay.
'very' becomes 'v' (abbreviation).
'old partner' becomes 'ex' (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'no' becomes 'n' (common abbreviation).
'good' becomes 'g' (abbreviation).
'v'+'ex'+'i'+'n'+'g'='VEXING'
(Other definitions for vexing that I've seen before include "Annoying, worrying" , "Irking" , "Extremely annoying" , "Causing irritation" .)