1970s music, say, is constant in party (5)
I believe the answer is:
disco
'1970s music' is the definition.
I can't tell whether this definition defines the answer.
'is constant in party' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'in' indicates putting letters inside.
'party' becomes 'do' (do is a kind of party).
'is'+'c'='isc'
'isc' going inside 'do' is 'DISCO'.
'say' is the link.
Can you help me to learn more?
(Other definitions for disco that I've seen before include "Dance club with recorded music" , "party for dancers" , "Public dance hall with recorded music" , "Club or party with dancing" , "Party with records" .)