Second eleven? It may be let off when there is bad language (5)
I believe the answer is:
steam
'it may be let off when there is bad language' is the definition.
The definition suggests a singular noun which matches the answer.
'second eleven?' is the wordplay.
'second' becomes 's' (common abbreviation).
'eleven?' becomes 'team' (eleven or XI can mean a football team).
's'+'team'='STEAM'
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Water at boiling temperature diffused in the atmosphere" , "Sort of water" , "Engine driver" , "Momentum - boiled water" , "Kind of power" .)