Complex network holds one source of oil (5)
I believe the answer is:
maize
'source of oil' is the definition.
(I've seen this before)
'complex network holds one' is the wordplay.
'complex network' becomes 'maze' (I've seen this before).
'holds' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'maze' placed around 'i' is 'MAIZE'.
(Other definitions for maize that I've seen before include "Something cultivated" , "Indian corn" , "Oil producer" , "Corncob plant" , "Sweet corn" .)