Conclude it’s worse to leave port (5)
I believe the answer is:
infer
'conclude' is the definition.
(inferring is a kind of concluding)
'worse to leave port' is the wordplay.
'worse' becomes 'inferior'.
'to leave' indicates named letters should be taken away.
'port' becomes 'rio' (I've seen this before).
'inferior' with 'rio' taken away is 'INFER'.
'it's' is the link.
(Other definitions for infer that I've seen before include "Derive by reasoning" , "Derive as a consequence" , "Conclude or judge from facts" , "come to a conclusion" , "Deduce, surmise" .)