Block angry school principal (9)

I believe the answer is:

crosshead

'block' is the definition.
(crosshead is a kind of block)

'angry school principal' is the wordplay.
'angry' becomes 'cross' (I've seen this before).
'school principal' becomes 'head' ('head' can be a synonym of 'school principal').
'cross'+'head'='CROSSHEAD'

I've seen this clue in The Independent.

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