Take away warship on land (8)
I believe the answer is:
subtract
'take away' is the definition.
(thesaurus)
'warship on land' is the wordplay.
'warship' becomes 'sub' (sub is a kind of warship).
'on' means one lot of letters go next to another.
'land' becomes 'tract' (I've seen this in other clues**).
'sub'+'tract'='SUBTRACT'
(Other definitions for subtract that I've seen before include "dock" , "Take away (a number)" , "Withdraw" , "Deduct" , "Take an amount away" .)