Sea change, as fish enters drink (5-2)
I believe the answer is:
shake up
'sea change' is the definition.
'shake up' can be an answer for 'change' (I have seen 'Fundamental change ' mean 'shake up' so perhaps 'change' could also mean 'shake up'). I'm not sure about the 'sea' bit.
'fish enters drink' is the wordplay.
'fish' becomes 'hake' (hake is a kind of fish).
'enters' is an insertion indicator.
'drink' becomes 'sup' (to sup is to drink).
'hake' inserted inside 'sup' is 'SHAKE-UP'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for shake up that I've seen before include "Disturb" , "Drastic reorganisation" , "Shock" , "Fundamental change" , "Major reorganisation" .)