The spare you have in a bag to go swimming (4)
I believe the answer is:
thin
'the spare' is the definition.
(I've seen this in another clue)
'you have in a bag to go swimming' is the wordplay.
I cannot really see how this works, but
'in' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for thin that I've seen before include "Sparse; not dense" , "Made of light material" , "Implausible" , "Fine" , "as 7 [LOAF] may be sliced" .)