He investigates passing round pickle sandwiches (7)
I believe the answer is:
coroner
'he investigates passing' is the definition.
(a coroner investigates someone's death or passing)
'round pickle sandwiches' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'pickle' becomes 'corner' (both can mean a problem or difficulty).
'sandwiches' indicates putting letters inside (goes on either side like a sandwich).
'o' going inside 'corner' is 'CORONER'.
(Other definitions for coroner that I've seen before include "One investigating death" , "Inquest holder" , "Official enquiring into any death" , "Official inquiring into deaths not by natural causes" , "Official holding inquests" .)