Excited by a delay about holding mass (8)
I believe the answer is:
animated
'excited' is the definition.
(animating is a kind of exciting)
'a delay about holding mass' is the wordplay.
'delay' becomes 'detain' ('detain' can be a synonym of 'delay'**).
'about' says the letters should be written backwards.
'holding' means one lot of letters goes inside another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'detain' reversed gives 'niated'.
'niated' going around 'm' is 'nimated'.
'a'+'nimated'='ANIMATED'
'by' acts as a link.
(Other definitions for animated that I've seen before include "put in spirit" , "'Lively, quick (8)'" , "Interesting and lively - as a cartoon film is" , "Mandate I got to be lively" , "Diamante (anag)" .)