Encounter a fashionable dog (5)
I believe the answer is:
incur
'encounter' is the definition.
The answer and definition can be both to do with fundamental existence as well as being verbs in their base form.
Maybe you can see a link between them that I can't see?
'a fashionable dog' is the wordplay.
'a fashionable' becomes 'in' (in can mean fashionable or trendy).
'dog' becomes 'cur' (both can mean a despicable person).
'in'+'cur'='INCUR'
Can you help me to learn more?
(Other definitions for incur that I've seen before include "Become liable to (eg, a penalty)" , "Bring upon oneself - be liable for" , "Invite, bring about" , "Become liable to (something) through one's actions" , "Become liable or subject to" .)