Direction to players — Desdemona extremely upset, Iago devious (6)
I believe the answer is:
adagio
'direction to players' is the definition.
(musical instruction to play slowly)
'desdemona extremely upset iago devious' is the wordplay.
'extremely' suggests removing the centre.
'upset' is a reversal indicator.
'devious' indicates anagramming the letters.
'desdemona' with its middle taken out is 'da'.
'da' reversed gives 'ad'.
'iago' with letters rearranged gives 'agio'.
'ad'+'agio'='ADAGIO'
(Other definitions for adagio that I've seen before include "Slow piece of music" , "develops slowly" , "(A movement) to be played slowly" , "In slow time (mus.)" , "Musical composition to be played slowly and gracefully" .)