Do exercises in bad shape to begin with — they won't bring much joy (5,7)
I believe the answer is:
party poopers
'won't bring much joy' is the definition.
I can't tell whether this definition defines the answer.
'do exercises in bad shape to begin with they' is the wordplay.
I cannot quite understand how this works, but
'do' could be 'party' (I've seen this before) and 'party' is found within the answer.
'exercises' could be 'pe' (Physical Education class) and 'pe' is located in the answer.
'bad' could be 'poor' (similar in meaning) and 'poor' is located in the remaining letters.
A single letter 's' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for party poopers that I've seen is " Do spoilers".)