Odd drip at small outhouse has decreased (10)
I believe the answer is:
diminished
'decreased' is the definition.
('diminish' can be a synonym of 'decrease')
'odd drip at small outhouse' is the wordplay.
'odd' means one should take alternating letters.
'at' means one lot of letters go next to another.
'small' becomes 'mini' (short for miniature).
'outhouse' becomes 'shed' (a shed is an outside building).
The alternating letters of 'drip' are 'di'.
'di'+'mini'+'shed'='DIMINISHED'
'has' acts as a link.
(Other definitions for diminished that I've seen before include "Lessened" , "Became less" , "reduced" , "Not so impressive" , "weakened" .)