A park with character, mister (4)
I believe the answer is:
hyde
'a park' is the definition.
(I've seen this before)
'with character mister' is the wordplay.
'with' means one lot of letters goes inside another.
'character' becomes 'd'.
'mister' becomes 'hye' (I can't explain this - if you can you should give a lot more credence to this answer).
'd' placed into 'hye' is 'HYDE'.
Can you help me to learn more?
(Other definitions for hyde that I've seen before include "First president" , "and 21 Across: Green area in London" , "Manchester town, London park" , "... the latter side of [EVIL] nature" , "See 15" .)