For example, this is left in signal (4)
I believe the answer is:
clue
'for example this' is the definition.
(this clue)
'left in signal' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'signal' becomes 'cue' (as in an actor's cue).
'l' inserted within 'cue' is 'CLUE'.
'is' acts as a link.
(Other definitions for clue that I've seen before include "One of twenty-four in this puzzle" , "Pointer" , "Intimation (like this?)" , "Hint, tip" , "Aid to detection" .)