Errant mates possibly 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 cannot see how one could define the other.
'errant mates' is the wordplay.
'errant' indicates anagramming the letters (errant can mean travelling or straying).
'mates' is an anagram of 'STEAM'.
'possibly' is the link.
Can you help me to learn more?
(Other definitions for steam that I've seen before include "comes with a hiss?" , "Vaporised water" , "Old-fashioned power" , "Like old radio" , "Gas from boiliing water" .)