Wake when shaken; it's mid-afternoon (4)
I believe the answer is:
stir
'wake' is the definition.
(thesaurus)
'shaken it's mid-afternoon' is the wordplay.
'shaken' indicates anagramming the letters.
'mid' says to take the centre.
The central letter of 'afternoon' is 'r'.
'its' is an anagram of 'sti'.
'sti'+'r'='STIR'
'when' is the link.
(Other definitions for stir that I've seen before include "Prison - mix" , "Shake up" , "Agitate, arouse" , "Create revolutionary movement" , "Produce disorder < , wake up" .)