Recurring musical phrase (9)
(Other definitions for leitmotiv that I've seen before include "Limit vote (anag.)" , "Musical theme" , "Recurring literary theme" , "repetitive theme" , "recurrent idea" .)
'recurring musical phrase' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for motto that I've seen before include "Phrase in a Christmas cracker" , "Short sentence adopted as rule of conduct" , "Watchword" , "Guiding adage" , "cry" .)