In this rebus puzzle, you add and subtract the letters from the words together to make new words.
ACORN – CORN = A[CORN] = A
N + COMBINE – COMB = N[COMB]INE = NINE
LEG + OTTER – GO = LE[GO]TTER = LETTER
UNDO + SCARER – OSCAR + WAITER – I = UND[OSCAR]ERWA[I]TER = UNDERWATER
SPOOL – POOL + HIP = S[POOL]HIP = SHIP
This gives the phrase A nine-letter underwater ship, which is of course a SUBMARINE.