Terrible conceit, ergo, endlessly selfish (10)
I believe the answer is:
egocentric
'selfish' is the definition.
(thesaurus)
'terrible conceit ergo endlessly' is the wordplay.
'terrible' indicates an anagram.
'endlessly' means to remove the last letter.
'conceit'+'ergo'='conceitergo'
'conceitergo' with its last letter removed is 'conceiterg'.
'conceiterg' is an anagram of 'EGOCENTRIC'.
(Other definitions for egocentric that I've seen before include "Self-centred person with little regard for others" , "Self-seeking" , "Not thinking of others" , "Self-interested" , "Self-absorbed" .)