Comparatively clean river in the outskirts of Peshawar (5)
I believe the answer is:
purer
'comparatively clean' is the definition.
('pure' can be similar in meaning to 'clean')
'river in the outskirts of peshawar' is the wordplay.
'river' becomes 'ure' (River Ure).
'in the' means one lot of letters goes inside another.
'outskirts of' suggests removing the centre (only letters on the outskirts).
'peshawar' with its centre removed is 'pr'.
'ure' inserted within 'pr' is 'PURER'.
(Other definitions for purer that I've seen before include "Less immoral" , "more clean" , "More chaste" , "cleaner" , "Less adulterated" .)