Not quick to react (5)
I believe the answer is:
inert
'not quick to react' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for inert that I've seen before include "Devoid of motion or change" , "Like some gases" , "Slow and apathetic" , "(Chem.) not reactive" , "Unreactive; still" .)