Leave a narrow opening across top of pipe (5)
I believe the answer is:
split
'leave' is the definition.
(thesaurus)
'a narrow opening across top of pipe' is the wordplay.
'a narrow opening' becomes 'slit' (I've seen this before).
'across' indicates putting letters inside.
'top of' indicates taking the first letters.
The initial letter of 'pipe' is 'p'.
'slit' placed around 'p' is 'SPLIT'.
(Other definitions for split that I've seen before include "Break, fracture" , "Separate, break into parts" , "somewhere on the Adriatic?" , "ice cream concoction" , "Cleavage" .)