Brace front of tank with third of props (3)
I believe the answer is:
two
'brace' is the definition.
(I've seen this in another clue)
'front of tank with third of props' is the wordplay.
'front of' says to take the initial letters (the letter at the front of the word).
'third of props' becomes 'o' (3rd letter of 'props').
The initial letters of 'tank with' is 'tw'.
'tw'+'o'='TWO'
(Other definitions for two that I've seen before include "Item" , "Old song 'Tea for . . .'" , "'It takes ? to tango'" , "Pair, deuce" , "Cardinal number" .)