Certainly, marines get out of shape (6)

I believe the answer is:

deform

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'shape' is the definition.
(deforming is a kind of shaping)

'certainly marines get out of' is the wordplay.
'certainly' becomes 'de' (I can't justify this - if you can you should give a lot more credence to this answer).
'marines' becomes 'RM' (abbreviation for 'Royal Marines').
'get' is an insertion indicator.
'out' is an anagram indicator.
'de'+'rm'='derm'
'of' is an anagram of 'fo'.
'derm' going around 'fo' is 'DEFORM'.

Can you help me to learn more?

(Other definitions for deform that I've seen before include "Distort out of shape" , "Pervert" , "Contort" , "Twist out of shape" , "buckle" .)

I've seen this clue in The Independent.

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