Chap set a switch incorrectly: pay attention in future! (5,4,5)
I believe the answer is:
watch this space
'pay attention in future' is the definition.
('watch this space' means wait for new developments)
'chap set a switch incorrectly' is the wordplay.
'incorrectly' indicates anagramming the letters (I've seen this in other clues).
'chap'+'set'+'a'+'switch'='chapsetaswitch'
'chapsetaswitch' with letters rearranged gives 'WATCH THIS SPACE'.
(Other definitions for watch this space that I've seen before include "Details to follow" , "more information later" , "Look at what I'm doing with room" .)