Dates ruined by variable boyfriend? (6)
I believe the answer is:
steady
'boyfriend?' is the definition.
(I have seen 'Regular boyfriend' mean 'steady' so perhaps 'boyfriend' could also mean 'steady')
'dates ruined by variable' is the wordplay.
'ruined' indicates anagramming the letters.
'by' means one lot of letters go next to another.
'variable' becomes 'y' (algebra).
'dates' is an anagram of 'stead'.
'stead'+'y'='STEADY'
(Other definitions for steady that I've seen before include "Slow and ... wins the race" , "Uniform" , "date seen regularly" , "warning to keep calm" , "Stable or regular" .)