Plain couple crossing river “discovered” pier (7)
I believe the answer is:
prairie
'plain' is the definition.
(I've seen this before)
'couple crossing river discovered pier' is the wordplay.
'couple' becomes 'pair' (synonyms).
'crossing' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'discovered' says to take the centre (dis-covered - outer letters removed).
The centre of 'pier' is 'ie'.
'pair' going around 'r' is 'prair'.
'prair'+'ie'='PRAIRIE'
(Other definitions for prairie that I've seen before include "Cowboy song: ` `Bury me not on the lone .......' '" , "Treeless grassy plain, in the old West perhaps" , "Extensive US grassland" , "North American grassland" , "Schooners once crossed it" .)