Impractical if fine vessel capsizes, blocking cross-channel one (7)
I believe the answer is:
utopian
'impractical' is the definition.
('utopian' can be similar in meaning to 'impractical')
'fine vessel capsizes blocking cross-channel one' is the wordplay.
'fine' becomes 'ai' (I've seen this in other clues).
'vessel' becomes 'pot' (pot is a kind of vessel**).
'capsizes' is a reversal indicator (I've seen 'capsized' mean this).
'blocking' means one lot of letters goes inside another.
'cross-channel one' becomes 'un' ('one' in French).
'ai'+'pot'='aipot'
'aipot' in reverse letter order is 'topia'.
'topia' inserted inside 'un' is 'UTOPIAN'.
'if' is the link.
(Other definitions for utopian that I've seen before include "in a dreamy state" , "Too perfect for this world" , "Thomas More-ish?" , "Aspiring to impracticable perfection" , "More work was literally so" .)