Problem is hard, leading to irritation (5)
I believe the answer is:
hitch
'problem' is the definition.
(I've seen this before)
'hard leading to irritation' is the wordplay.
'hard' becomes 'h'.
'leading to' is a charade indicator (letters next to each other) (some letters go before or lead others).
'irritation' becomes 'itch' (I've seen this before).
'h'+'itch'='HITCH'
'is' acts as a link.
(Other definitions for hitch that I've seen before include "interruption" , "Unforeseen obstacle" , "Hindrance" , "Temporary difficulty" , "Tether (rope); difficulty" .)