Guide by a light outside (4)
I believe the answer is:
lead
'guide' is the definition.
(lead can mean to usher or guide)
'a light outside' is the wordplay.
'light' becomes 'led' (abbreviation for light emitting diode).
'outside' is an insertion indicator.
'a' put into 'led' is 'LEAD'.
'by' acts as a link.
(Other definitions for lead that I've seen before include "Sort of poisoning" , "Chief performer" , "Primary" , "Travel in front of" , "in first place" .)