Saint's half-hearted prohibition (5)
I believe the answer is:
alban
'saint's' is the definition.
(St Alban)
'half-hearted prohibition' is the wordplay.
'hearted' means to look at the middle letters.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
The middle of 'half' is 'al'.
'al'+'ban'='ALBAN'
(Other definitions for alban that I've seen before include "Saint who was first British Christian martyr" , "First British martyr, supposedly born in Verulamium" , "early martyr" , "Hertfordshire saint" , "First Christian martyr in England" .)