Worm found by queen once on hat (7)
I believe the answer is:
annelid
'worm' is the definition.
(annelid worm is a kind of worm)
'by queen once on hat' is the wordplay.
'by' means one lot of letters go next to another.
'queen once' becomes 'anne'.
'on hat' becomes 'lid' ('lid' can be a synonym of 'hat'. I am not sure about the 'on' bit.).
'anne' put next to 'lid' is 'ANNELID'.
'found' is the link.
Can you help me to learn more?
(Other definitions for annelid that I've seen before include "Earthworm or leech" , "Lindane (anag.); a kind of worm" , "Class of worms" , "leech, perhaps" , "leech, for example" .)