Captain beginning to steam fish (7)
I believe the answer is:
skipper
'captain' is the definition.
(synonyms)
'beginning to steam fish' is the wordplay.
'beginning to' indicates taking the first letters.
'fish' becomes 'kipper' (type of fish).
The initial letter of 'steam' is 's'.
's'+'kipper'='SKIPPER'
(Other definitions for skipper that I've seen before include "delicate winger?" , "Captain of team or boat - type of butterfly" , "Captain - one using a rope" , "Head" , "one that's put over the side" .)