Unfortunate lapses following treatment introduced by hospital (7)
I believe the answer is:
hapless
'unfortunate' is the definition.
(similar in meaning)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for hapless that I've seen before include "Pathetic" , "He slaps (anag) - unlucky" , "Ill-fated" , "Piteous" , "Ill-starred" .)