Suspect smack and, initially, grass (5,1,3)
I believe the answer is:
smell a rat
'suspect' is the definition.
(to smell a rat is to suspect something)
'smack and initially grass' is the wordplay.
I cannot really see how this works, but
'grass' could be 'rat' (both can mean a snitch or informer) and 'rat' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for smell a rat that I've seen before include "Find something fishy" , "Suspect something's amiss" , "Suspect deception" , "Become suspicious" , "Suspect something wrong" .)