It's difficult for a motherless dog! (5)
I believe the answer is:
stiff
This could be a double definition.
'it's difficult' is the first definition.
(I've seen this before)
'motherless dog' is the second definition.
'stiff' can be an answer for 'dog' (thesaurus). I'm not sure about the 'motherless' bit.
'for a' is the link.
Can you help me to learn more?
(Other definitions for stiff that I've seen before include "tough" , "Frigid" , "Rigid, prim" , "Inflexible" , "Formal" .)