Awfully tired intellectual becomes harmful (11)
I believe the answer is:
detrimental
'harmful' is the definition.
(I've seen this before)
'awfully tired intellectual' is the wordplay.
'awfully' indicates anagramming the letters.
'intellectual' becomes 'mental' ('mental' can be similar in meaning to 'intellectual'**).
'tired' with letters rearranged gives 'detri'.
'detri'+'mental'='DETRIMENTAL'
'becomes' is the link.
(Other definitions for detrimental that I've seen before include "not helpful" , "Damaging" , "which is bad for one" , "Tending to cause harm" , "Causing harm" .)