Be careful! That's the boyfriend! (6)
I believe the answer is:
steady
'the boyfriend' is the definition.
(I have seen 'Regular boyfriend' mean 'steady' so perhaps 'boyfriend' could also mean 'steady')
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for steady that I've seen before include "Stable, not shaky" , "Regular, dependable" , "One's regular boyfriend" , "Poised" , "Stable or regular" .)