Working in MPH, AA meet required speed (11)
I believe the answer is:
amphetamine
'speed' is the definition.
(thesaurus)
'working in mph aa meet' is the wordplay.
'working' indicates an anagram.
'in'+'mph'+'aa'+'meet'='inmphaameet'
'inmphaameet' is an anagram of 'AMPHETAMINE'.
'required' acts as a link.
(Other definitions for amphetamine that I've seen before include "Mood-altering drug" , "speed that's restricted by law" , "Upper" , "Stimulant drug" .)