Fix geometric ratio with any number (3)
I believe the answer is:
pin
'fix' is the definition.
(to pin is to fix in place)
'geometric ratio with any number' is the wordplay.
'geometric ratio' becomes 'pi' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'pi'+'n'='PIN'
Can you help me to learn more?
(Other definitions for pin that I've seen before include "form of identification" , "Peg or nail" , "Stick with flag placed in golf hole" , "Tack; fasten" , "Small pointed metal fastener" .)