Animal's acceleration between satisfactory and very good (5)
I believe the answer is:
okapi
'animal's' is the definition.
(African mammal)
'acceleration between satisfactory and very good' is the wordplay.
'acceleration' becomes 'a' (symbol used in Physics e.g. f=ma).
'between' means one lot of letters goes inside another.
'satisfactory' becomes 'ok' (similar in meaning).
'and' says to put letters next to each other.
'very good' becomes 'pi' (old-fashioned term for 'pious').
'ok'+'pi'='okpi'
'a' inserted within 'okpi' is 'OKAPI'.
(Other definitions for okapi that I've seen before include "Giraffe family member" , "Rare mammal" , "African relative of the giraffe" , "Ruminant related to the giraffe" , "African mammal related to giraffe" .)