Occasionally thin, dry spruce (4)
I believe the answer is:
tidy
'spruce' is the definition.
(similar in meaning)
'occasionally thin dry' is the wordplay.
'occasionally' means one should take alternating letters.
'thin'+'dry'='thindry'
The alternating letters of 'thindry' are 'TIDY'.
(Other definitions for tidy that I've seen before include "Neat - container for odds and ends" , "Spick-and-span" , "Spruce up" , "so substantial a sum" , "generous" .)