To hug madly is difficult (5)
I believe the answer is:
tough
'difficult' is the definition.
(similar in meaning)
'to hug madly' is the wordplay.
'madly' indicates an anagram.
'hug' anagrammed gives 'ugh'.
'to'+'ugh'='TOUGH'
'is' is the link.
(Other definitions for tough that I've seen before include "Hard to cut or chew" , "Challenging" , "Strict, uncompromising" , "Rugged" , "Strong - difficult" .)