Idle American Eileen regularly ignored heartless sharks (7)
I believe the answer is:
useless
'idle' is the definition.
(I've seen this before)
'american eileen regularly ignored heartless sharks' is the wordplay.
'american' becomes 'US' (United States).
'regularly ignored' means one should take alternating letters (letters removed at regular intervals).
'heartless' suggests removing the centre.
'sharks' with its middle taken out is 'ss'.
The alternate letters of 'eileen' are 'ele'.
'us'+'ele'+'ss'='USELESS'
(Other definitions for useless that I've seen before include "Without any worthwhile application" , "Serving no practical purpose" , "Far from impressive" , "Having no benefical function" , "Of no practical or worthwhile application" .)