Huge tank carrying first of soldiers (4)
I believe the answer is:
vast
'huge' is the definition.
(I've seen this before)
'tank carrying first of soldiers' is the wordplay.
'tank' becomes 'vat' (I've seen this before).
'carrying' indicates putting letters inside.
'first of' says to take the initial letters.
The initial letter of 'soldiers' is 's'.
'vat' enclosing 's' is 'VAST'.
(Other definitions for vast that I've seen before include "Colossal" , "Boundless" , "Astronomical" , "Extremely large or extensive" , "Of very great extent" .)