A second spat, not finished ... a recurring theme (5)
I believe the answer is:
motif
'a recurring theme' is the definition.
(I've seen this before)
'a second spat not finished' is the wordplay.
'a second' becomes 'mo' (both are short periods of time).
'spat' becomes 'tiff' (synonyms).
'not finished' means to remove the last letter.
'tiff' with its final letter taken off is 'tif'.
'mo'+'tif'='MOTIF'
(Other definitions for motif that I've seen before include "Unifying idea in an artistic work" , "Repeated theme" , "Logo -- pattern" , "Recurring image in design" , "Theme in a work of art" .)