Two legs, either side of one head (5)
I believe the answer is:
onion
'head' is the definition.
(informal term for someone's head or brain)
'two legs either side of one' is the wordplay.
'two legs' can mean two replacements for 'leg'.
'leg' becomes 'on' (cricket term, fielding positions).
'leg' becomes 'on' (cricket term, fielding positions).
'either side of' is an insertion indicator (one word goes around another one).
'one' becomes 'i' (Roman numeral).
'on'+'on'='onon'
'onon' going around 'i' is 'ONION'.
(Other definitions for onion that I've seen before include "Eye-watering vegetable" , "Aromatic flavourful versatile vegetable" , "Type of allium" , "Cause of tears" , "Strong tasting vegetable" .)