Taxi driver's bottom pierced by tip of elephant trunk (5)
I believe the answer is:
caber
'trunk' is the definition.
(I've seen this before)
'taxi driver's bottom pierced by tip of elephant' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'bottom' suggests the final letters.
'pierced by' means one lot of letters goes inside another.
'tip of' says to take the initial letters.
The final letter of 'driver' is 'r'.
The first letter of 'elephant' is 'e'.
'cab'+'r'='cabr'
'cabr' enclosing 'e' is 'CABER'.
(Other definitions for caber that I've seen before include "It's tossed in Scotland" , "Item tossed in Highland sport" , "Tree trunk for tossing" , "Pole needed for game up north" , "Thrown tree trunk" .)