Company returning leading American sea creature (7)
I believe the answer is:
octopus
'sea creature' is the definition.
(I've seen this in another clue)
'company returning leading american' is the wordplay.
'company' becomes 'co' (abbreviation).
'returning' is a reversal indicator.
'leading' becomes 'top' (both can mean first-class).
'american' becomes 'US' (United States).
'co' written backwards gives 'oc'.
'oc'+'top'+'us'='OCTOPUS'
(Other definitions for octopus that I've seen before include "Mollusc with long tentacles" , "Tentacled cephalopod" , "person with widespread influence" , "Invertebrate sea creature with three hearts" , "large, influential organization" .)