Guard to be dispatched before I and Len return (8)
I believe the answer is:
sentinel
'guard' is the definition.
(sentinel is a kind of guard)
'dispatched before i and len return' is the wordplay.
'dispatched' becomes 'sent' (I've seen this before).
'before' says to put letters next to each other.
'and' says to put letters next to each other.
'return' is a reversal indicator.
'len' back-to-front is 'nel'.
'sent'+'i'+'nel'='SENTINEL'
'to be' is the link.
(Other definitions for sentinel that I've seen before include "Sentry" , "Guardsman" , "Guard - ten lines (anag)" , "Soldier who stands and keeps watch" , "Look out" .)