Simple pal in trouble (5)
I believe the answer is:
plain
'simple' is the definition.
(similar in meaning)
'pal in trouble' is the wordplay.
'trouble' indicates anagramming the letters.
'pal'+'in'='palin'
'palin' with letters rearranged gives 'PLAIN'.
(Other definitions for plain that I've seen before include "Simple, ordinary" , "Clear - open - grassland" , "Unadorned on flat area" , "Unflavoured" , "Unvarnished" .)