Become fashionable, you understand (5,2)
I believe the answer is:
catch on
'become fashionable' is the definition.
(I've seen this before)
'you understand' is the wordplay.
I cannot quite see how this works, but
'understand' could be 'catch' (I've seen this before) and 'catch' is located in the answer.
The remaining letters 'on' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for catch on that I've seen before include "twig" , "Informally, become popular or come to understand" , "Understand - become fashionable" , "Get it" .)