Immobilise one cover raised in Peru's old national gallery (12)
I believe the answer is:
incapacitate
'immobilise one cover' is the definition.
'incapacitate' can be an answer for 'immobilise' (thesaurus). I am not sure about the 'one cover' bit.
'raised in peru's old national gallery' is the wordplay.
'raised' becomes 'paci' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'peru's old national' becomes 'Inca' (the Inca were a Peruvian civilisation).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'inca'+'tate'='incatate'
'paci' going into 'incatate' is 'INCAPACITATE'.
(Other definitions for incapacitate that I've seen before include "Prevent from functioning normally" , "Take power away from" , "Disable; disqualify (law)" .)