Uphill task, introducing a new motto (6)
I believe the answer is:
slogan
'motto' is the definition.
(synonyms)
'uphill task introducing a new' is the wordplay.
'uphill task' becomes 'slog'.
'introducing' says to put letters next to each other.
'a new' becomes 'an' (I've seen this in other clues).
'slog'+'an'='SLOGAN'
(Other definitions for slogan that I've seen before include "'Motto, watchword (6)'" , "War cry - advertising jingle" , "Battle cry - advertising phrase" , "Tag line" , "Logo" .)