It grows underground at mid-April (5)
I believe the answer is:
tuber
'it grows underground' is the definition.
(I've seen this in another clue)
'at mid-april' is the wordplay.
'at' becomes 'tube' (I can't justify this - if you can you should believe this answer much more).
'mid' says to take the centre (mid- can mean the middle of something).
The central letter of 'april' is 'r'.
'tube'+'r'='TUBER'
Can you help me to learn more?
(Other definitions for tuber that I've seen before include "One goes underground" , "or still vegetating?" , "Rhizome, as of potato" , "A brute of a spud" , "Fleshy underground part of a stem, often used as food" .)