Numb bottom following half-hour back massage (3)
I believe the answer is:
rub
'massage' is the definition.
(I've seen this before)
'numb bottom following half-hour back' is the wordplay.
'bottom' indicates one should take the final letters.
'following' says to put letters next to each other.
'half' indicates taking half.
'back' is a reversal indicator.
The final letter of 'numb' is 'b'.
'hour' cut in half is 'ur'.
'b'+'ur'='bur'
'bur' written backwards gives 'RUB'.
(Other definitions for rub that I've seen before include "Advice to Aladdin" , "Ay, there's the - (Hamlet)" , "Chafe; difficulty" , "Polish - difficulty" , "Cause friction" .)