A boyfriend in the firm (6)
I believe the answer is:
steady
I believe this is a double definition.
'a boyfriend' is the first definition.
(I have seen 'Regular boyfriend' mean 'steady' so perhaps 'boyfriend' could also mean 'steady')
'the firm' is the second definition.
(I've seen this in another clue)
'in' is the link.
(Other definitions for steady that I've seen before include "Well balanced and stable" , "Customary" , "Not liable to fluctuation" , "Well-balanced" , "Ready . . . . . . go" .)