Sentence on introduction of yellow elastic (8)
I believe the answer is:
stretchy
'elastic' is the definition.
(similar in meaning)
'sentence on introduction of yellow' is the wordplay.
'sentence' becomes 'stretch' (a stretch in prison).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'introduction of' says to take the initial letters.
The first letter of 'yellow' is 'y'.
'stretch'+'y'='STRETCHY'
(Other definitions for stretchy that I've seen before include "Elastic" , "Flexible" , "Of variable length, perhaps" .)