Larks circling a goat-legged deity (4)
I believe the answer is:
faun
'goat-legged deity' is the definition.
(I've seen this in another clue)
'larks circling a' is the wordplay.
'larks' becomes 'fun' (I am not sure about this - if you are sure you should believe this answer much more).
'circling' is an insertion indicator (circling can mean going around or containing).
'fun' enclosing 'a' is 'FAUN'.
Can you help me to learn more?
(Other definitions for faun that I've seen before include "Rural deity, half-man, half-goat" , "Ancient Italian deity with goat-like features" , "Mr Tumnus or his ilk" , "Roman deity, in the form of a man with a goat's horns, legs and tail" , "This old deity" .)