Very early or mid-April, possibly (10)
I believe the answer is:
primordial
'very early' is the definition.
'primordial' can be an answer for 'early' (similar in meaning). I am not sure about the 'very' bit.
'or mid-april possibly' is the wordplay.
'possibly' indicates an anagram.
'or'+'mid'+'april'='ormidapril'
'ormidapril' is an anagram of 'PRIMORDIAL'.
Can you help me to learn more?
(Other definitions for primordial that I've seen before include "Having existed from the very start - limp or arid (anag)" , "Original" , "from the first" , "From the beginning of time" , "Existing from the start of time" .)