Fathers rage when boarding steamship (5)
I believe the answer is:
sires
'fathers' is the definition.
('sire' can be a synonym of 'father')
'rage when boarding steamship' is the wordplay.
'rage' becomes 'ire' (ire is anger).
'when boarding' is an insertion indicator.
'steamship' becomes 'ss' (nautical abbreviation).
'ire' put into 'ss' is 'SIRES'.
(Other definitions for sires that I've seen before include "Fathers" , "Begets" .)