Slow blast with Queen in comeback (6)
I believe the answer is:
retard
'slow' is the definition.
('retard' can be a synonym of 'slow')
'blast with queen in comeback' is the wordplay.
'blast' becomes 'drat' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'in comeback' shows that the letters should be reversed in order.
'drat'+'er'='drater'
'drater' written backwards gives 'RETARD'.
(Other definitions for retard that I've seen before include "Keep back, make slow" , "Slow down for trader" , "Darted about to hold back or delay" , "Hold back (development)" , "Hold back, slow progress" .)