Small boat at the same time capsized (3)
I believe the answer is:
tub
'small boat' is the definition.
'tub' can be an answer for 'boat' (tub could historically mean a small boat). I'm not sure about the 'small' bit.
'at the same time capsized' is the wordplay.
'at' means one lot of letters go next to another.
'the same' becomes 'bu' (I am not sure about this - if you are sure you should believe this answer much more).
'time' becomes 't' (abbreviation).
'capsized' says the letters should be written in reverse (in a down clue, the word turns upside down).
'bu' put next to 't' is 'but'.
'but' in reverse letter order is 'TUB'.
Can you help me to learn more?
(Other definitions for tub that I've seen before include "Small cask" , "A ranting preacher informally thumps it" , "Container for yogurt, ice-cream" , "Pot (of e.g. ice-cream)" , "An old-fashioned bath" .)