1: Hold onto string and file (5)
2: Wild rose engulfs one willow (5)
3: Spin broken shingle (7)
4: Put a new value on what secret agent is hiding (5)
5: Moderately amphibian, for example, by the middle of the creek (2,1,6)
6: Always, the final way is a mountain! (7)
7: Clip of Southern Conservative is very revealing at first (7)
8: Shoos stray housecats (6 3)
12: Win, or find an heir (9)
13: Heard Italian city in canyon is from the stone age, perhaps (3-6)
15: Tar mess contaminated rivers (7)
16: He stirs up a one who had referred to him in the past. (7)
17: Strange guy, Nero jr. (7)
19: Speak slowly, with upward lilt at the beginning (5)
20: Scrabble piece having D and a diacritical mark (5)
21: Scotsman turns to notice drifter (5)