Wireless device turned on before interval (5)
I believe the answer is:
pager
'wireless device' is the definition.
(communications device)
'turned on before interval' is the wordplay.
'turned' says the letters should be written in reverse.
'on' becomes 're' ('on' can be mean 'with reference to').
'before' says to put letters next to each other.
'interval' becomes 'gap' (I've seen this in another clue).
're'+'gap'='regap'
'regap' backwards is 'PAGER'.
(Other definitions for pager that I've seen before include "Bleeper device" , "Way to contact" , "Electronic alerting device" , "Radio alert device" , "Electronic messaging device" .)