Is able to be led astray by a light (6)
I believe the answer is:
candle
'a light' is the definition.
(candle is a kind of light)
'is able to be led astray by' is the wordplay.
'is able to' becomes 'can' (if you can do something you are able to do it).
'astray' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'led' anagrammed gives 'dle'.
'can'+'dle'='CANDLE'
(Other definitions for candle that I've seen before include "that's useful if power's cut" , "Means of illumination" , "Artificial light" , "Old form of illumination" , "Wax or tallow light" .)