Clue: Being shelled, one commando unfortunately gets into difficulty (9)
Answer: ARMADILLO
Explanation: Definition: Being shelled
definition: a being (a creature) that is shelled 
A="one" + RM (Royal Marine, "commando"), plus ILL="unfortunately" inside ADO="difficulty"
 
ILL can be an adverb e.g. 'the rain was ill timed' / 'it was unfortunately timed'