It's difficult for a motherless dog! (5)
I believe the answer is:
stiff
This clue 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 am not certain of 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" .)