Go underground, we're told, to park in London (4)
I believe the answer is:
hyde
'park in london' is the definition.
(I've seen this in another clue)
'go underground we're told' is the wordplay.
'go underground' becomes 'hide' (I've seen this in another clue).
'we're told' indicates a 'sounds like' (homophone) clue.
'hide' is a homophone of 'HYDE'.
'to' is the link.
(Other definitions for hyde that I've seen before include "First president" , "... the latter side of [EVIL] nature" , "doctor turned into during work" , "Jekyll's alter ego" , "and 21 Across: Green area in London" .)