Is tense with animal on back (6)
I believe the answer is:
assist
'back' is the definition.
(thesaurus)
'is tense with animal' is the wordplay.
'tense' becomes 't' (grammatical abbreviation).
'with' means one lot of letters go next to another.
'animal' becomes 'ass' (term for a donkey).
'is'+'t'='ist'
'ist' put after 'ass' is 'ASSIST'.
'on' acts as a link.
(Other definitions for assist that I've seen before include "Help, lend a hand" , "provide backing" , "Support" , "Help member" , "Be helpful to" .)