Useless letter to Ian, perhaps (4)
I believe the answer is:
vain
'useless' is the definition.
(I've seen this before)
'letter to ian perhaps' is the wordplay.
'letter' becomes 'v' ().
'perhaps' is an anagram indicator.
'ian' with letters rearranged gives 'ain'.
'v'+'ain'='VAIN'
(Other definitions for vain that I've seen before include "Conceited or pointless" , "Having too much pride in one's appearance" , "It's no good" , "Stuck-up? It's hopeless" , "without success" .)