Notice doctor sip ale (6)
I believe the answer is:
espial
'notice' is the definition.
(I've seen this before)
'doctor sip ale' is the wordplay.
'doctor' indicates an anagram.
'sip'+'ale'='sipale'
'sipale' with letters rearranged gives 'ESPIAL'.
(Other definitions for espial that I've seen before include "Spotting" , "look" , "Seeing" , "notice" , "Detection" .)