Equipment put round most of the perimeter (5)
I believe the answer is:
girth
'perimeter' is the definition.
(girth is a kind of perimeter)
'equipment put round most of the' is the wordplay.
'equipment put' becomes 'rig' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'round' says the letters should be written backwards.
'most of' means to remove the last letter.
'rig' in reverse letter order is 'gir'.
'the' with its final letter taken off is 'th'.
'gir'+'th'='GIRTH'
Can you help me to learn more?
(Other definitions for girth that I've seen before include "all the way round" , "A band passing under horse's belly" , "Circumpherence" , "Band around a horse's belly" , "Distance around, often of the tummy" .)