When it's soft and terribly sore, there's a problem (5)
I believe the answer is:
poser
'there's a problem' is the definition.
(poser is a kind of problem)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for poser that I've seen before include "One pretends to be something they are not" , "Sticky problem" , "Question - a sitter" , "Person still being portrayed" , "Pseud" .)