Reveal a tennis term to a cricket side (3,2)
I believe the answer is:
let on
'reveal' is the definition.
(I know that reveal can be written as let on)
'a tennis term to a cricket side' is the wordplay.
'a tennis term' becomes 'let' (I've seen this in another clue).
'to a cricket side' becomes 'on' (cricket term, on or leg side).
'let'+'on'='LET ON'
(Other definitions for let on that I've seen before include "Allow to board, or pretend" , "Pretend informally or permit to board" , "Reveal, divulge" , "Pretend, feign" , "Divulge info; pretend" .)