Gritty mature lady left in former Arab union (8)
I believe the answer is:
granular
'gritty' is the definition.
(I know that gritty can be written as granular)
'mature lady left in former arab union' is the wordplay.
'mature lady' becomes 'gran' (grandmother).
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'former arab union' becomes 'UAR' (United Arab Republic, former country).
'l' inserted into 'uar' is 'ular'.
'gran'+'ular'='GRANULAR'
(Other definitions for granular that I've seen before include "Particulate" , "Made of small particles" , "Consisting of small hard particles" , "Gritty (like some sugar?)" , "Like grains or hard particles" .)