Part of army besieging University College (6)
I believe the answer is:
corpus
'college' is the definition.
(I've seen this before)
'part of army besieging university' is the wordplay.
'part of' means one lot of letters goes inside another.
'army besieging' becomes 'corps' (I am not sure about this - if you are sure you should believe this answer much more).
'university' becomes 'U' (abbreviation for university).
'corps' enclosing 'u' is 'CORPUS'.
Can you help me to learn more?
(Other definitions for corpus that I've seen before include "Body of writing" , "Body (of work etc)" , "collection of literature" , "The entire works of a writer" , "A body as of writings" .)