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' ('in' can mean fashionable).
'dog' becomes 'cur' (a cur is an aggressive or scruffy 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" .)