See-through underwear in series beginning to split, check trousers (6,4)
I believe the answer is:
string vest
'see-through underwear' is the definition.
(type of see-through underwear)
'series beginning to split check trousers' is the wordplay.
I cannot quite understand how this works, but
'series' could be 'string' (sequence of similar items or events) and 'string' is located in the answer.
The remaining letters 'vest' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for string vest that I've seen before include "Top" , "CLOTHING" , "Open-weave undergarment" , "Item of underwear" .)