When damaged, hold out for the end of the line (5)
I believe the answer is:
holed
'when damaged' is the definition.
Although both the answer and definition are past participle verbs, I cannot see how they can define each other.
'hold out for the end of the line' is the wordplay.
'out' means to remove the last letter.
'for' means one lot of letters go next to another.
'the end of the line' becomes 'ed' (I can't explain this - if you can you should give a lot more credence to this answer).
'hold' with its final letter removed is 'hol'.
'hol'+'ed'='HOLED'
Can you help me to learn more?
(Other definitions for holed that I've seen before include "Perforated" , "Pierced side" , "Likely to sink" , "Scuttled" , "after being hit by torpedo" .)