Pro got excited in bed with head of department - that's not allowed! (9)
I believe the answer is:
forbidden
'not allowed' is the definition.
(I've seen this before)
'pro got excited in bed with head of department' is the wordplay.
'pro' becomes 'for' (I've seen this before).
'got excited' is an anagram indicator (I've seen 'getting excited' mean this).
'with' indicates putting letters inside.
'head of' indicates taking the first letters.
The initial letter of 'department' is 'd'.
'in'+'bed'='inbed'
'inbed' is an anagram of 'biden'.
'biden' going around 'd' is 'bidden'.
'for'+'bidden'='FORBIDDEN'
'that's' is the link.
(Other definitions for forbidden that I've seen before include "Disallowed, banned" , "This fruit is always more enticing" , "Proscribed" , "Not allowed, banned" , "Verboten, prohibited" .)