Boys out of bounds in middle of narrow ravine (6)
I believe the answer is:
arroyo
'ravine' is the definition.
(thesaurus)
'boys out of bounds in middle of narrow' is the wordplay.
'out of bounds' says to take the centre (the bounds or edges of the word are removed).
'in' means one lot of letters goes inside another.
'middle of' says to take the centre.
The centre of 'boys' is 'oy'.
The centre of 'narrow' is 'arro'.
'oy' placed into 'arro' is 'ARROYO'.
(Other definitions for arroyo that I've seen before include "Rocky ravine" , "Dry gully; yo roar (anag.)" , "Often dry desert ravine" , "Dry watercourse" , "Dry river gully" .)