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' means one lot of letters go next to another.
'key' becomes 'c' (musical key).
'performance' becomes 'act' (act is a kind of performance).
'c'+'act'='cact'
'i' 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" .)