Middle of April or earlier (5)
I believe the answer is:
prior
'earlier' is the definition.
(synonyms)
'middle of april or' is the wordplay.
'middle of' says to take the centre.
The centre of 'april' is 'pri'.
'pri'+'or'='PRIOR'
(Other definitions for prior that I've seen before include "head of a religious community" , "former conviction" , "Erstwhile" , "Antecedent" , "Existing beforehand" .)