Cross to put outside old Islington pub (7)
I believe the answer is:
tangelo
'cross' is the definition.
(I've seen this before)
'to put outside old islington pub' is the wordplay.
'put outside' is an insertion indicator.
'old islington pub' becomes 'Angel' (historical pub in Islington).
'to' going around 'angel' is 'TANGELO'.
(Other definitions for tangelo that I've seen before include "Citrus fruit hybrid" , "sharp cross" , "fruitful merger?" , "one cross, sour" , "Mixed fruit" .)