Look after silicon storage unit (4)
I believe the answer is:
silo
'storage unit' is the definition.
(I've seen this before)
'look after silicon' is the wordplay.
'look' becomes 'lo' (archaic term meaning 'look!').
'after' means one lot of letters go next to another.
'silicon' becomes 'Si' (Si is the chemical symbol for silicon).
'lo' after 'si' is 'SILO'.
(Other definitions for silo that I've seen before include "Grain storage area" , "Cylindrical storage tower" , "Store for grain or fodder" , "Missile launcher; farm store" , "Missile pit" .)