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