Thanks ixion and congrats, peterm
I must admit that I discounted your effort, as I thought that the grammar didn't work - the plural of "sprat" is "sprats", so I thought "sprat encourages" was correct, and hence ST URGES ON" implied. However, after seeing the popularity of the clue, I Googled and did find a couple of references to "sprat" being used as a plural.
Possibly I was being a bit picky anyway!