What's old in phony antique? Solve that! (8)
I believe the answer is:
equation
'solve that' is the definition.
(I know that equations can be solved)
'what's old in phony antique?' is the wordplay.
'what's old' becomes 'o' (common abbreviation eg in OE for Old English).
'in' means one lot of letters goes inside another.
'phony' is an anagram indicator (a phony or false spelling).
'antique' with letters rearranged gives 'equatin'.
'o' placed into 'equatin' is 'EQUATION'.
(Other definitions for equation that I've seen before include "Mathematically, two are equal" , "feature of maths" , "Ambrose --, author of The Devil's Dictionary" , "Mathematical statement that two things are equal" , "It can be algebraic" .)