Old, quaint engineer resolved mathematical statement (8)
I believe the answer is:
equation
'mathematical statement' is the definition.
(equation is a kind of mathematical statement)
'old quaint engineer resolved' is the wordplay.
I cannot really see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
an anagram of 'quaint' is 'quatin' which is located in the left over letters.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for equation that I've seen before include "E=mc2, say" , "Mathemical statement using = sign" , "feature of maths" , "Situation where many factors need consideration" , "Maths statement with =" .)