Wet weather dogging opening of Test series (5)
I believe the answer is:
train
'test series' is the definition.
'train' can be an answer for 'series' (train is a kind of series). I am unsure of the 'test' bit.
'wet weather dogging opening' is the wordplay.
'wet weather' becomes 'rain' (rain is wet weather).
'dogging' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'opening' means one lot of letters go next to another (some letters appear before or \'open\' others).
'rain' after 't' is 'TRAIN'.
'of' is the link.
Can you help me to learn more?
(Other definitions for train that I've seen before include "Procession" , "Do exercises" , "Series (of gears); coach" , "Rail service" , "School - transport" .)