Short excursion into byway to find loo (7)
I believe the answer is:
latrine
'loo' is the definition.
(I know that latrine is a type of loo)
'short excursion into byway' is the wordplay.
'short' means to remove the last letter.
'excursion' becomes 'trip' (a trip is an excursion or outing).
'into' is an insertion indicator.
'byway' becomes 'lane' (I've seen this before).
'trip' with its final letter removed is 'tri'.
'tri' going into 'lane' is 'LATRINE'.
'to find' is the link.
(Other definitions for latrine that I've seen before include "An army loo" , "where to go in the army" , "Primitive toilet" , "office" , "Lavatory (in barracks)" .)