Slim airhead managed a boat (8)
I believe the answer is:
trimaran
'a boat' is the definition.
(trimaran is a kind of sailing vessel)
'slim airhead managed' is the wordplay.
'slim' becomes 'trim' (both can mean skinny or slight).
'airhead' becomes 'a' (head letter of 'air').
'managed' becomes 'ran' (both can mean organised or was in charge of).
'trim'+'a'+'ran'='TRIMARAN'
(Other definitions for trimaran that I've seen before include "Sailing boat with three hulls" , "type of boat" , "Sailing vessel" , "Three-hulled vessel" , "Three-hulled boat" .)