Secret store of headgear used up round start of spring (5)
I believe the answer is:
stash
'secret store' is the definition.
(I've seen this before)
'headgear used up round start of spring' is the wordplay.
'headgear' becomes 'hat' (hat is a kind of headgear).
'used' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'up' is a reversal indicator (in down clue: letters go upwards).
'round' means one lot of letters goes inside another.
'start of' suggests taking the first letters.
The initial letter of 'spring' is 's'.
'hat'+'s'='hats'
'hats' backwards is 'stah'.
'stah' going around 's' is 'STASH'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for stash that I've seen before include "drug store" , "(Hidden store of) drugs" , "Secret store of valuables" , "Hide (something) away" , "Secret(ly) store" .)