Climbing mountains, don't mention that drop of rain (6)
I believe the answer is:
splash
'drop of rain' is the definition.
'splash' can be an answer for 'drop' (I've seen this before). I'm unsure of the 'of rain' bit.
'climbing mountains don't mention' is the wordplay.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'mountains' becomes 'alps' (European mountain range).
'don't mention' becomes 'sh' (I can't explain this - if you can you should give a lot more credence to this answer).
'alps' written backwards gives 'spla'.
'spla'+'sh'='SPLASH'
'that' is the link.
Can you help me to learn more?
(Other definitions for splash that I've seen before include "Drop" , "major news item" , "Big media display -- bellyflop effect" , "Effect of body falling on water" , "Effect of jumping into water" .)