Identify mistake in dosage (8)
I believe the answer is:
diagnose
'identify' is the definition.
(thesaurus)
'mistake in dosage' is the wordplay.
'mistake' indicates anagramming the letters.
'in'+'dosage'='indosage'
'indosage' is an anagram of 'DIAGNOSE'.
(Other definitions for diagnose that I've seen before include "San Diego (anag)" , "Examine and identify (problem)" , "Find cause of illness in San Diego" , "Identify a disease from its symptoms" , "Establish nature of illness after study of symptoms" .)