Sea creature, head in focus, heading off (7)
I believe the answer is:
octopus
'sea creature' is the definition.
(I've seen this before)
'head in focus heading off' is the wordplay.
'head' becomes 'top' (synonyms).
'in' indicates putting letters inside.
'heading off' means to remove the first letter.
'focus' with its first letter taken away is 'ocus'.
'top' put within 'ocus' is 'OCTOPUS'.
(Other definitions for octopus that I've seen before include "Influential person" , "Cephalopod with suckered tentacles" , "Invertebrate sea creature with three hearts" , "Tentacled sea creature" , "something from La Mer?" .)