Plants one after key performance (5)
I believe the answer is:
cacti
'plants' is the definition.
(cactus is a kind of plant)
'one after key performance' is the wordplay.
'one' becomes 'i' (Roman numeral).
'after' is a charade indicator (letters next to each other).
'key' becomes 'c' (musical key).
'performance' becomes 'act' (act is a kind of performance).
'c'+'act'='cact'
'i' put after 'cact' is 'CACTI'.
(Other definitions for cacti that I've seen before include "Plants native to arid regions" , "tuna, for example" , "They're decidedly prickly" , "Desert features" , "Features of westerns" .)