Monstrous woman therefore capsized steamship (6)
I believe the answer is:
ogress
'monstrous woman' is the definition.
(female ogre)
'therefore capsized steamship' is the wordplay.
'therefore' becomes 'ergo' (I've seen this in another clue).
'capsized' is a reversal indicator (in a down clue, the word turns upside down).
'steamship' becomes 'ss' (nautical abbreviation).
'ergo' written backwards gives 'ogre'.
'ogre'+'ss'='OGRESS'
(Other definitions for ogress that I've seen before include "Female man-eating giant" , "Giant woman" , "Particular nasty woman" , "Terrifying female" , "Some progressive" .)