Old-fashioned person strangely thinks I'm cute and prudent at heart (5-2-3-3)
I believe the answer is:
stick-in-the-mud
'old-fashioned person' is the definition.
'stick-in-the-mud' can be an answer for 'person' (stick-in-the-mud is a kind of person). I'm not certain of the 'old-fashioned' bit.
'strangely thinks i'm cute and prudent at heart' is the wordplay.
'strangely' indicates an anagram.
'and' means one lot of letters go next to another.
'prudent at heart' becomes 'd' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'thinks'+'im'+'cute'='thinksimcute'
'thinksimcute' anagrammed gives 'stickinthemu'.
'stickinthemu'+'d'='STICK-IN-THE-MUD'
Can you help me to learn more?
(Other definitions for stick-in-the-mud that I've seen before include "One who resists change" , "Completely unenterprising person" , "Staid or predictable conservative person" , "Old fogey" , "One who won't get moving" .)