Problem for model being a show-off (5)
I believe the answer is:
poser
This clue is a triple definition!
'problem' is the first definition.
(I know that poser is a type of problem)
'for model' is the second definition.
(synonyms)
'being' is a link.
'a show-off' is the third definition.
(poser is a kind of show-off)
(Other definitions for poser that I've seen before include "Tricky clue" , "Problem set by the model" , "Artist's model" , "Puzzling problem" , "Difficult problem" .)