Outdoor area, one surrounded by three men? (4)
I believe the answer is:
park
'outdoor area' is the definition.
'one surrounded by three men?' is the wordplay.
'three men?' means three substitutions for 'man'.
'one' becomes 'a'.
'surrounded by' indicates putting letters inside.
'man' becomes 'p' (abbreviation for pawn, a chess piece or 'man').
'man' becomes 'r' (abbreviation for rook, a chess piece or 'man'**).
'man' becomes 'k' (abbreviation for king, a chess man).
'p'+'r'='pr'
'a' put inside 'pr' is 'par'.
'par'+'k'='PARK'
(Other definitions for park that I've seen before include "Area with trees - leave the car" , "Enclosed area with trees" , "See 8" , "Green or Hyde" , "Position a car" .)