Sheepish communicants? (3,4)

I believe the answer is:

pen pals

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

But I can't explain it at all! Could you help me understand?

(Other definitions for pen pals that I've seen before include "They correspond" , "Young correspondents" , "Friends who communicate in writing" , "'Friends through correspondence (3,4)'" .)

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