Clue: Horse 'as to be 'eld up (7)
Answer: MUSTANG
Explanation: Horse 'as to be 'eld up (7)
MUST 'ANG 
I don't think the first apostrophe is required (there's no reason why it can't be "has to") as it is the indicator for MUST, and it is the " 'eld up" part which equates to 'ANG