A seven-day period with crumbs for bird (7)
I believe the answer is:
goshawk
'bird' is the definition.
(goshawk is a kind of bird)
'a seven-day period with crumbs' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'period' could be 'go' (go is a kind of period) and 'go' is located in the answer.
'with' could be 'w' (abbreviation) and 'w' is located in the answer.
This explanation may well be incorrect...
'for' is the link.
Can you help me to learn more?
(Other definitions for goshawk that I've seen before include "Bird used in falconry" , "Short-winged bird of prey" .)