Democrats broaching alternative to Thatcher? There’s just a small catch (7)
I believe the answer is:
tiddler
'just a small catch' is the definition.
(term for a small fish)
I can't explain the remainder of the clue.
(Other definitions for tiddler that I've seen before include "Small fish (or child?)" , "Little chap" , "Tiny fish, child" , "A small fish children try to catch" , "Minnow" .)