Plunge in Mersey cut short after one minute (7)
I believe the answer is:
immerse
'plunge in' is the definition.
(I've seen this before)
'mersey cut short after one minute' is the wordplay.
'cut short' means to remove the last letter.
'after' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'minute' becomes 'm' (abbreviation).
'mersey' with its last letter taken off is 'merse'.
'i'+'m'='im'
'merse' put after 'im' is 'IMMERSE'.
(Other definitions for immerse that I've seen before include "Absorb" , "Plunge deep into liquid" , "Involve deeply" , "duck" , "Dunk" .)