Crossword Genius

One held to be over your head, Mrs? (4)

Ross

I believe the answer is:

gamp

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'your head mrs?' is the definition.
I can't tell whether this defines the answer.

'one held to be' is the wordplay.
'one' becomes 'gp' (I can't explain this - if you can you should give a lot more credence to this answer).
'held' means one lot of letters goes inside another.
'to be' becomes 'am'.
'gp' going around 'am' is 'GAMP'.

'over' is the link.

Can you help me to learn more?

(Other definitions for gamp that I've seen before include "Waterproof cover" , "Midwife" , "Umbrella (inf)" , "Umbrella (from Dickens character)" .)

I've seen this clue in the Evening Standard.
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