Key followed by old singer (4)
I believe the answer is:
alto
'singer' is the definition.
(alto is a kind of singer)
'key followed by old' is the wordplay.
'key' becomes 'alt' (I've seen this in other clues).
'followed by' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'alt'+'o'='ALTO'
(Other definitions for alto that I've seen before include "The highest adult male singing-voice" , "Member of choir" , "Key old > male who scores high" , "someone to sing" , "Woman's singing voice" .)