Let alumni say nothing about head of Latin (8)
I believe the answer is:
obstacle
'let' is the definition.
(I've seen this before)
'alumni say nothing about head of latin' is the wordplay.
'alumni' becomes 'obs' (abbreviation for old boys).
'say nothing' becomes 'tace' (Latin word meaning 'be silent').
'about' indicates putting letters inside.
'head of' says to take the initial letters.
The initial letter of 'latin' is 'l'.
'tace' placed around 'l' is 'tacle'.
'obs'+'tacle'='OBSTACLE'
(Other definitions for obstacle that I've seen before include "Maybe let" , "Progress blocker" , "Anything that stands in the way" , "Thing blocking progress" , "Stab Cleo (anag) - which hinders progress" .)