Terrible head of department with study almost chock-a-block (8)
I believe the answer is:
dreadful
'terrible' is the definition.
(I know that terrible can be written as dreadful)
'head of department with study almost chock-a-block' is the wordplay.
I cannot quite understand how this works, but
'department' could be 'd' (abbreviation eg in MOD Ministry of Defence) and 'd' is found in the answer.
'study' could be 'read' (synonyms) and 'read' is found within the answer.
The remaining letters 'ful' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for dreadful that I've seen before include "Informally very bad, perhaps the weather" , "Horrible, shocking" , "Appalling, awful" , "Shockingly bad" , "Atrocious, awful" .)