I've an alternative that's simple (5)
I believe the answer is:
naive
'simple' is the definition.
('naive' can be similar in meaning to 'simple')
'i've an alternative' is the wordplay.
'alternative' is an anagram indicator.
'ive'+'an'='ivean'
'ivean' with letters rearranged gives 'NAIVE'.
'that's' is the link.
(Other definitions for naive that I've seen before include "Artless, innocent" , "Ingenuous and gullible" , "Lacking judgment" , "Dewy-eyed" , "Unenlightened" .)