The speed in G&S irritates (6)
I believe the answer is:
grates
'irritates' is the definition.
(grating is a kind of irritating)
'the speed in gs' is the wordplay.
'the speed' becomes 'rate' (rate is a kind of speed**).
'in' is an insertion indicator.
'rate' placed within 'gs' is 'GRATES'.
(Other definitions for grates that I've seen before include "Rubs harshly against, perhaps on nerves" , "Makes rasping sound, hard on ear" , "Rasps or irritates" , "Makes harsh sound by rubbing" , "Rasps against, food or nerves" .)