Such underwear as gets many in a terrible lather? (7)
I believe the answer is:
thermal
'such underwear as' is the definition.
I can't judge whether this definition defines the answer.
'many in a terrible lather?' is the wordplay.
'many' becomes 'm'.
'in' is an insertion indicator.
'a terrible' indicates anagramming the letters.
'lather' with letters rearranged gives 'theral'.
'm' placed inside 'theral' is 'THERMAL'.
'gets' acts as a link.
Can you help me to learn more?
(Other definitions for thermal that I've seen before include "type of baths" , "Relating to heat" , "help for glider" , "Upward current of warm air" , "Pertaining to heat" .)