The one-piece swimsuits all open like parts of the bikini (4)
I believe the answer is:
tops
'the one-piece swimsuits all open' is the definition.
I can't judge whether this defines the answer.
'parts of the bikini' is the wordplay.
'parts of' says to take the initial letters (I've seen 'part' mean this).
'bikini' becomes 'ops' (I can't justify this - if you can you should believe this answer much more).
The first letter of 'the' is 't'.
't'+'ops'='TOPS'
'like' acts as a link.
Can you help me to learn more?
(Other definitions for tops that I've seen before include "Beats" , "Max - spinning toys" , "The best" , "Caps; outdoes" , "Betters" .)