Secret store of extremely soft wood (5)
I believe the answer is:
stash
'secret store' is the definition.
(I've seen this before)
'extremely soft wood' is the wordplay.
'extremely' suggests removing the centre.
'wood' becomes 'ash' (I've seen this before**).
'soft' with its middle removed is 'st'.
'st'+'ash'='STASH'
'of' acts as a link.
(Other definitions for stash that I've seen before include "(Hidden store of) drugs" , "Cache" , "Treasure trove" , "Squirrel away" , "drug store" .)