Headgear, say, on the counter outside small store (5)
I believe the answer is:
stash
'store' is the definition.
(stash is a kind of store)
'headgear say on the counter outside small' is the wordplay.
'headgear say' becomes 'hats' (I can't justify this - if you can you should give a lot more credence to this answer).
'on the counter' says the letters should be written backwards (counter can mean in the opposite direction).
'outside' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'hats' in reverse letter order is 'stah'.
'stah' placed around 's' is 'STASH'.
Can you help me to learn more?
(Other definitions for stash that I've seen before include "collection" , "Consignment of drugs" , "Squirrel away" , "Secret(ly) store" , "Secret hoard" .)