A head of physics, small, showing desire for food (8)
I believe the answer is:
appetite
'desire for food' is the definition.
(I've seen this before)
'a head of physics small' is the wordplay.
'head of' suggests taking the first letters.
'small' becomes 'petite' (similar in meaning).
The initial letter of 'physics' is 'p'.
'a'+'p'+'petite'='APPETITE'
'showing' is the link.
(Other definitions for appetite that I've seen before include "'Strong desire, craving (8)'" , "Hunger or zeal" , "Natural craving" , "stomach" , "Physical desire as for food" .)