Ponder taking drug to institute constant harmony (5)
I believe the answer is:
music
'harmony' is the definition.
(thesaurus)
'ponder taking drug to institute constant' is the wordplay.
'ponder' becomes 'muse' ('muse' can be a synonym of 'ponder').
'taking' indicates named letters should be taken away.
'drug' becomes 'e' (e is another name for ecstasy).
'to institute' becomes 'i' (common abbreviation as in MIT).
'constant' becomes 'c' (mathematical abbreviation).
'muse' with 'e' taken away is 'mus'.
'mus'+'i'+'c'='MUSIC'
(Other definitions for music that I've seen before include "graces?" , "Pleasing sounds" , "Crafted sound" , "Artistic form of auditory communication" , "Orsino's food of love (Twelfth Night)" .)