Lay oneself open to trendy dog (5)
I believe the answer is:
incur
'lay oneself open to' 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 "Enter into" , "Invite, bring about" , "To become liable to, suffer" , "Become subject to (eg, taxes)" , "Become subject to (penalty)" .)