Ocean rover at sea holding it up in extreme response (12)
I believe the answer is:
overreaction
'extreme response' is the definition.
(I've seen this before)
'ocean rover at sea holding it up' is the wordplay.
'at sea' indicates anagramming the letters.
'holding' means one lot of letters goes inside another.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'it' backwards is 'ti'.
'ocean'+'rover'='oceanrover'
'oceanrover' anagrammed gives 'overreacon'.
'overreacon' going around 'ti' is 'OVERREACTION'.
'in' is the link.
(Other definitions for overreaction that I've seen before include "Extreme response" , "Excessive response" .)