Hard to catch foursome in English vessel (7)
I believe the answer is:
evasive
'hard to catch' is the definition.
(I've seen this before)
'foursome in english vessel' is the wordplay.
'foursome' becomes 'IV' (synonyms).
'in' means one lot of letters goes inside another.
'english' becomes 'e' (abbreviation).
'vessel' becomes 'vase' (vase is a kind of vessel).
'e'+'vase'='evase'
'iv' inserted into 'evase' is 'EVASIVE'.
(Other definitions for evasive that I've seen before include "Not frank or responsive" , "Avoiding the issue" , "Dissembling" , "Uncommitted" , "Devious" .)