Free service not available (3,3)
I believe the answer is:
let off
'free' is the definition.
(letting off is a kind of freeing)
'service not available' is the wordplay.
'service' becomes 'let' (let is a kind of service).
'not available' becomes 'off' (I've seen this in other clues**).
'let'+'off'='LET OFF'
(Other definitions for let off that I've seen before include "Set free without charge" , "Pardon - discharge" , "Detonate" , "given a reprieve" , "Not punished at all" .)