Guilt makes head of biology feeble (5)
I believe the answer is:
blame
'guilt' is the definition.
(thesaurus)
'head of biology feeble' is the wordplay.
'head of' says to take the initial letters.
'feeble' becomes 'lame' (lame can mean feeble or pathetic).
The initial letter of 'biology' is 'b'.
'b'+'lame'='BLAME'
'makes' acts as a link.
(Other definitions for blame that I've seen before include "Condemnation" , "Accuse" , "Assign responsibility for misdeed" , "Impute fault to" , "Attribute fault or guilt to" .)