Demonstrate royal pony-tail is wet? (7)
I believe the answer is:
showery
'is wet?' is the definition.
(similar in meaning)
'demonstrate royal pony-tail' is the wordplay.
'demonstrate' becomes 'show' (synonyms).
'royal' becomes 'er' (abbreviation for Elizabeth Regina).
'tail' indicates one should take the final letters.
The final letter of 'pony' is 'y'.
'show'+'er'+'y'='SHOWERY'
(Other definitions for showery that I've seen before include "Raining intermittently" , "far from fine" , "some rain" , "Like many British days" , "Wet" .)