Under spruce, hole's opening (5)
I believe the answer is:
neath
'under' is the definition.
(I've seen this in another clue)
'spruce hole's opening' is the wordplay.
'spruce' becomes 'neat' (synonyms).
'opening' says to take the initial letters.
The initial letter of 'holes' is 'h'.
'neat'+'h'='NEATH'
(Other definitions for neath that I've seen before include "Welsh industrial town" , "River" , "Town near Swansea" , "Under, poetically" , "not on top" .)