Keep alien in wet conditions (6)
I believe the answer is:
retain
'keep' is the definition.
(retaining is a kind of keeping)
'alien in wet conditions' is the wordplay.
'alien' becomes 'et' (extraterrestrial).
'in' indicates putting letters inside.
'wet conditions' becomes 'rain' (I can't explain this - if you can you should give a lot more credence to this answer).
'et' inserted inside 'rain' is 'RETAIN'.
Can you help me to learn more?
(Other definitions for retain that I've seen before include "Keep in Tirane" , "Keep; remember" , "Continue to hold on to" , "'Hold, keep back (6)'" , "Hold onto" .)