Expression of frustration about a ring road somewhere in Hertfordshire (7)
I believe the answer is:
watford
'somewhere in hertfordshire' is the definition.
(town in Hertfordshire)
'expression of frustration about a ring road' is the wordplay.
'expression of frustration' becomes 'wtf' (abbreviation for 'what the f***').
'about' is an insertion indicator.
'ring' becomes 'o' (letter resembling a ring).
'road' becomes 'rd' (abbreviation).
'wtf' placed around 'a' is 'watf'.
'watf'+'o'+'rd'='WATFORD'
(Other definitions for watford that I've seen before include "Town north of London" , "Hertfordshire town" , "Town NW of London, within the M 25" .)