Harmonised short anthem penned by revolutionary (6)
I believe the answer is:
rhymed
'harmonised' is the definition.
(I've seen this before)
'short anthem penned by revolutionary' is the wordplay.
'short' means to remove the last letter.
'anthem' becomes 'hymn' ('hymn' can be a synonym of 'anthem').
'penned by' indicates putting letters inside.
'revolutionary' becomes 'red'.
'hymn' with its final letter removed is 'hym'.
'hym' placed into 'red' is 'RHYMED'.
(Other definitions for rhymed that I've seen before include "that U2 did with The Who" , "Harmonised" , "Of verse, had similarity of sound in line endings" , "Like much verse" , "Made verses" .)