Naughty mates can be let off (5)
I believe the answer is:
steam
'let off' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how they can define each other.
'naughty mates' is the wordplay.
'naughty' is an anagram indicator.
'mates' anagrammed gives 'STEAM'.
'can be' acts as a link.
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Source of energy" , "Type of power" , "fume" , "Invisible gas from boiling water" , "letter opener?" .)