Being shelled, one commando unfortunately gets into difficulty (9)
I believe the answer is:
armadillo
'being shelled' is the definition.
(type of being or animal with a shell)
'one commando unfortunately gets into difficulty' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'commando' becomes 'RM' (abbreviation for Royal Marine).
'unfortunately' becomes 'ill' (as in ill-fated).
'gets into' indicates putting letters inside.
'difficulty' becomes 'ado' (ado is bother or difficulty).
'ill' put into 'ado' is 'adillo'.
'a'+'rm'+'adillo'='ARMADILLO'
(Other definitions for armadillo that I've seen before include "Nocturnal creature" , "Toothless Native American" , "Mammal with protective plates" , "Insectivorous mammal" , "animal on the loose!" .)