Argonaut crew on for crossing lake (7)
I believe the answer is:
telamon
'argonaut' is the definition.
(one of the Argonauts in Greek mythology)
'crew on for crossing lake' is the wordplay.
'crew' becomes 'team' (both can mean a group of colleagues).
'for crossing' is an insertion indicator (some letters go around or cross others).
'lake' becomes 'L' (geographical abbreviation).
'team'+'on'='teamon'
'teamon' enclosing 'l' is 'TELAMON'.
(Other definitions for telamon that I've seen before include "male supporter" , "supporting pillar in the shape of a man" , "Atlas" , "Column" .)