On top of everything else, dispatcher loses script (4)

I believe the answer is:

head

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'on top' is the definition.
'head' can be an answer for 'top' (synonyms). I am unsure of the 'on' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for head that I've seen before include "Chief; go first" , "Chief; promontory" , "Leader; stream source" , "Skull" , "Beer foam; front" .)

I've seen this clue in The Irish Times and the Irish Times (Crosaire).

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