Enthusiasm of AB abandoning a port (4)
I believe the answer is:
brio
'enthusiasm' is the definition.
(thesaurus)
'ab abandoning a port' is the wordplay.
'abandoning' suggests deleting specific letters.
'port' becomes 'rio' (I've seen this before).
'ab' with 'a' removed is 'b'.
'b'+'rio'='BRIO'
'of' acts as a link.
(Other definitions for brio that I've seen before include "Dash or verve, from Italian" , "(Of music) spirit" , "Drive" , "In music, con ..... means with dash and verve" , "'Liveliness, vivacity (4)'" .)