Insubstantial, somewhat irksome? Agreed (6)
I believe the answer is:
meagre
'insubstantial' is the definition.
(I've seen this before)
'somewhat irksome? agreed' is the wordplay.
'somewhat' indicates a hidden word.
'MEAGRE' can be found hidden inside 'irksome agreed'.
(Other definitions for meagre that I've seen before include "Lacking in quantity or quality" , "weak" , "Scarce, not plentiful" , "Inadequate, scanty" , "deficient in amount" .)