Excursion with sweetheart is rubbish (5)
I believe the answer is:
tripe
'is rubbish' is the definition.
(both can mean nonsense)
'excursion with sweetheart' is the wordplay.
'excursion' becomes 'trip' (I've seen this in other clues).
'with' says to put letters next to each other.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'trip'+'e'='TRIPE'
(Other definitions for tripe that I've seen before include "Lining of the stomach of a cow as food" , "Ruminant's stomach, prepared for cooking" , "Ruminant's stomach lining" , "Rubbish (colloq.)" , "Swallowed" .)