Doctor sent girl, reliable (8)
I believe the answer is:
sterling
'reliable' is the definition.
'doctor sent girl' is the wordplay.
'doctor' is an anagram indicator.
'sent'+'girl'='sentgirl'
'sentgirl' is an anagram of 'STERLING'.
(Other definitions for sterling that I've seen before include "Of the highest qualiy" , "banking standard" , "British coinage" , "Excellent - British money" , "first-class quality" .)