Giant metal container holding tar, mostly (5)
I believe the answer is:
titan
'giant' is the definition.
('titan' can be a synonym of 'giant')
'metal container holding tar mostly' is the wordplay.
'metal container' becomes 'tin' (I've seen this before).
'holding' means one lot of letters goes inside another.
'mostly' means to remove the last letter.
'tar' with its last letter removed is 'ta'.
'tin' placed around 'ta' is 'TITAN'.
(Other definitions for titan that I've seen before include "enormous thing" , "Saturn's moon" , "An old Greek god - Atlas was one" , "Old giant" , "Huge(ly important) type" .)