Beneath tower, search frantically for jumper (13)
I believe the answer is:
steeplechaser
'jumper' is the definition.
(person or horse)
'beneath tower search frantically' is the wordplay.
'beneath' means one lot of letters go next to another (in a down clue).
'tower' becomes 'steeple' (steeple is a kind of tower).
'frantically' indicates anagramming the letters.
'search' anagrammed gives 'chaser'.
'steeple' next to 'chaser' is 'STEEPLECHASER'.
'for' acts as a link.
(Other definitions for steeplechaser that I've seen before include "Racehorse - athlete" , "Horse for jump races" , "Competitive runner" , "Horse that jumps" , "Racing horse" .)