Set tailors up on wet land (7)
I believe the answer is:
stiffen
'set tailors' is the definition.
'stiffen' can be an answer for 'set' (thesaurus). I'm not sure about the 'tailors' bit.
'up on wet land' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'on wet' becomes 'fits' (I can't justify this - if you can you should give a lot more credence to this answer).
'land' becomes 'fen' (fen is a kind of land).
'fits' reversed gives 'stif'.
'stif'+'fen'='STIFFEN'
Can you help me to learn more?
(Other definitions for stiffen that I've seen before include "Steel" , "Give strength to" , "more severe" , "Become more rigid" , "Make rigid or inflexible" .)