Tremble as friend puts pressure on one gallery (9)

I believe the answer is:

palpitate

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'tremble' is the definition.
(I know that palpitate is a more specific form of the action tremble)

'friend puts pressure on one gallery' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'puts pressure on' becomes 'pi' (I can't explain this - if you can you should believe this answer much more).
'one gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'pal'+'pi'+'tate'='PALPITATE'

'as' is the link.

Can you help me to learn more?

(Other definitions for palpitate that I've seen before include "Throb rapidly" , "quake" , "Beat abnormally quickly" , "Shake, tremble" , "Pulsate" .)

I've seen this clue in The Mirror.

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