Sea creature over sea expressed fantastic spout (7)
I believe the answer is:
octopus
'sea creature' is the definition.
(I've seen this before)
'over sea expressed fantastic spout' is the wordplay.
'over' becomes 'o' (cricket abbreviation**).
'expressed' indicates a 'sounds like' (homophone) clue.
'fantastic' is an anagram indicator.
'sea' sounds like 'c'.
'spout' with letters rearranged gives 'topus'.
'o'+'c'+'topus'='OCTOPUS'
(Other definitions for octopus that I've seen before include "Spineless creature" , "Large marine mollusc" , "Influential person" , "something from La Mer?" , "Eight-legged creature" .)