Working and working, holding one's head (5)
I believe the answer is:
onion
'head' is the definition.
(informal term for someone's head or brain)
'working and working holding one's' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'and' means one lot of letters go next to another.
'working' becomes 'on' (e.g. a machine is working or on).
'holding' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'on'+'i'='oni'
'on' after 'oni' is 'ONION'.
(Other definitions for onion that I've seen before include "Type of allium" , "Aromatic flavourful versatile vegetable" , "One may be set" , "associated with 26 [TRIPE] ?" , "Pungent vegetable" .)