Setter had cryptic clue earlier: 'father of geometry?' (6)
I believe the answer is:
euclid
'father of geometry?' is the definition.
(I've seen this before)
'setter had cryptic clue earlier' is the wordplay.
'setter had' becomes 'I'd' (I had - referring to the crossword setter).
'cryptic' indicates an anagram (a cryptic or obscure order of the letters).
'earlier' means one lot of letters go next to another.
'clue' anagrammed gives 'eucl'.
'id' after 'eucl' is 'EUCLID'.
(Other definitions for euclid that I've seen before include "Greek father of geometry" , "mathematician appears" , "Greek mathematician, 3rd century BC" , "Alexandrian mathematician" , "Greek philosopher" .)