Snake occasionally seen in beer joint (5)
I believe the answer is:
ankle
'joint' is the definition.
(ankle joint is a kind of joint)
'snake occasionally seen in beer' is the wordplay.
'occasionally' indicates alternate letters.
'seen in' is an insertion indicator.
'beer' becomes 'ale' (type of beer).
The alternating letters of 'snake' are 'nk'.
'nk' going into 'ale' is 'ANKLE'.
(Other definitions for ankle that I've seen before include "Foot-leg connector" , "Part of the leg between the foot and the shin" , "Lower leg joint" , "Major joint in lower limb" , "Location of the talus" .)