Endless horse manoeuvres to keep wobbly bum in shape (7)
I believe the answer is:
rhombus
'shape' is the definition.
(rhombus is a kind of shape)
'endless horse manoeuvres to keep wobbly bum' is the wordplay.
'endless' means to remove the last letter.
'manoeuvres' indicates anagramming the letters.
'to keep' indicates putting letters inside.
'wobbly' is an anagram indicator (I've seen this in other clues).
'horse' with its last letter taken off is 'hors'.
'hors' with letters rearranged gives 'rhos'.
'bum' with letters rearranged gives 'mbu'.
'rhos' placed around 'mbu' is 'RHOMBUS'.
'in' is the link.
(Other definitions for rhombus that I've seen before include "Four-sided figure" , "Diamond-shaped figure" , "Type of parallelogram" , "Oblique-angled parallelogram" , "Type of quadrilateral" .)