Reappear concerning mongrel (5)
I believe the answer is:
recur
'reappear' is the definition.
(thesaurus)
'concerning mongrel' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'mongrel' becomes 'cur' (synonyms).
're'+'cur'='RECUR'
(Other definitions for recur that I've seen before include "Happen again (5)" , "Come round again" , "Perseverate" , "come back to one's mind" , "Repeat" .)