Sustain trendy dog (5)
I believe the answer is:
incur
'sustain' is the definition.
(I've seen this before)
'trendy dog' is the wordplay.
'trendy' becomes 'in'.
'dog' becomes 'cur' (cur is a kind of dog).
'in'+'cur'='INCUR'
(Other definitions for incur that I've seen before include "Become subject to (something unwelcome)" , "To become liable to, suffer" , "Bring on oneself, say costs" , "Become liable to (eg, a penalty)" , "Sustain" .)