It runs wild in Wales! (5)

I believe the answer is:

swale

I'm a little stuck... Click here to teach me more about this clue!

'it runs' is the definition.
The definition suggests a singular noun which matches the answer.

'wild in wales' is the wordplay.
'wild in' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'wales' becomes 'wale' (synonyms).
's'+'wale'='SWALE'

Can you help me to learn more?

(Another definition for swale that I've seen is " Shade".)

Want a hint initially instead of a full solution? Install my app