Perfect to hide a long time inside (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'hide a long time inside' is the wordplay.
'inside' indicates a hidden word.
'IDEAL' is hidden in the letters of 'hide a long time'.
'to' is the link.
(Other definitions for ideal that I've seen before include "Standard of perfection or excellence" , "that can't be bettered?" , "faultless" , "Benchmark" , "(the best one can get)" .)