Sign of pollution around river provides tension (6)
I believe the answer is:
strain
'tension' is the definition.
(synonyms)
'sign of pollution around river' is the wordplay.
I cannot quite understand how this works, but
'river' could be 'r' (abbreviation) and 'r' is found within the answer.
'sign' could be 'stain' (stain is a kind of sign) and 'stain' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'provides' is the link.
Can you help me to learn more?
(Other definitions for strain that I've seen before include "variety" , "Great effort - sieve" , "Overtax; tension" , "song" , "Put through a perforated device to separate" .)