Spring frost after 2nd of April (5)
I believe the answer is:
prime
'spring' is the definition.
(thesaurus)
'frost after 2nd of april' is the wordplay.
'frost' becomes 'rime' (rime is frost by freezing water vapour).
'after' is a charade indicator (letters next to each other).
'2nd of april' becomes 'p' (2nd letter of 'april').
'rime' put after 'p' is 'PRIME'.
(Other definitions for prime that I've seen before include "Number with no factor but itself and 1" , "Fundamental - of highest quality" , "Maybe eleven" , "Main; indivisible number" , "Capital - of first importance" .)