One college involved in developing alder rootlet (7)
I believe the answer is:
radicle
'rootlet' is the definition.
(I've seen this in another clue)
'one college involved in developing alder' is the wordplay.
'one' becomes 'i' (Roman numeral).
'college' becomes 'c'.
'involved in' means one lot of letters goes inside another.
'developing' is an anagram indicator (letters developed into a new form).
'i'+'c'='ic'
'alder' is an anagram of 'radle'.
'ic' placed within 'radle' is 'RADICLE'.
(Other definitions for radicle that I've seen before include "part of plant" , "bit of a nerve" , "A little root" .)