Parking in middle of night beside river as an outdoor activity (7)
I believe the answer is:
camping
'an outdoor activity' is the definition.
'parking in middle of night beside river' is the wordplay.
'parking' becomes 'p' (abbreviation on road signs).
'middle of night' becomes 'G' (middle letter of 'night').
'beside' says to put letters next to each other (I've seen this in other clues).
'river' becomes 'cam' (river in Cambridge, England).
'p'+'in'+'g'='ping'
'ping' put after 'cam' is 'CAMPING'.
'as' acts as a link.
(Other definitions for camping that I've seen before include "appearing affected?" , "leisure activity" , "Bivouacking" , "Type of holiday" , "Living in a tent" .)