Fellow leads firm sailor on board (2-5)
I believe the answer is:
co-stars
'fellow leads' is the definition.
'co-stars' can be an answer for 'leads' (co-star is a kind of lead). I am unsure of the 'fellow' bit.
'firm sailor on board' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'sailor' becomes 'tar' (slang term for a sailor).
'on' is an insertion indicator.
'board' becomes 'ss' (I've seen this in other clues).
'tar' placed into 'ss' is 'stars'.
'co'+'stars'='CO-STARS'
Can you help me to learn more?
(Another definition for co-stars that I've seen is " Joint leading actors".)