Head of theology on drugs will show bit of leg (5)
I believe the answer is:
thigh
'bit of leg' is the definition.
('thigh' is part of 'leg')
'head of theology on drugs will' is the wordplay.
'head of' indicates taking the first letters.
'on drugs will' becomes 'high' (I can't justify this - if you can you should believe this answer much more).
The initial letter of 'theology' is 't'.
't'+'high'='THIGH'
'show' is the link.
Can you help me to learn more?
(Other definitions for thigh that I've seen before include "Part of member" , "Upper joint of the leg" , "Femoral leg part" , "This bone is the femur" , "Ham" .)