Hertfordshire town theatre space hosting fair (9)
I believe the answer is:
stevenage
'hertfordshire town' is the definition.
(I've seen this before)
'theatre space hosting fair' is the wordplay.
'theatre space' becomes 'stage' (theatrical stage).
'hosting' is an insertion indicator (inserted letters are welcomed in or hosted).
'fair' becomes 'even' (I've seen this before).
'stage' enclosing 'even' is 'STEVENAGE'.
(Other definitions for stevenage that I've seen before include "New Town to north of London" , "League One club" , "Herts town" , "Hertfordshire new town" , "Variety of tea" .)