Enter and catch informer packing to go (9)
I believe the answer is:
penetrate
'enter' is the definition.
(penetrating is a kind of entering)
'catch informer packing to go' is the wordplay.
'catch' becomes 'net' (netting is a kind of catching).
'informer' becomes 'rat' ('rat' can be a synonym of 'informer').
'packing' means one lot of letters goes inside another.
'to go' becomes 'pee' (both can mean to urinate).
'net'+'rat'='netrat'
'netrat' placed into 'pee' is 'PENETRATE'.
'and' is the link.
(Other definitions for penetrate that I've seen before include "Run through" , "Force way through, into" , "Go through" , "Pierce" , "Pass into or through" .)