Once he started his act there was no holding him (7)
I believe the answer is:
houdini
'once he started his act there was no holding him' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
(Other definitions for houdini that I've seen before include "Harry . . . . . . ., famed American escapologist" , "Hungarian-born American magician, d. 1926" , "Entertainer bound" , "tie-break ace" , "being given to escapism?" .)