Companion turned to catch gentleman's name (8)
I believe the answer is:
christen
'name' is the definition.
'companion turned to catch gentleman's' is the wordplay.
'companion' becomes 'ch' (I've seen this in other clues).
'turned' is a reversal indicator.
'to catch' becomes 'net' (netting is a kind of catching).
'gentleman' becomes 'sir' (I've seen this before).
'net'+'sir'='netsir'
'netsir' reversed gives 'risten'.
'ch'+'risten'='CHRISTEN'
(Other definitions for christen that I've seen before include "Give a name to (baby)" , "attach handle to" , "Baptise and name" , "Baptize" , "Name in church" .)