Promises plasma's first over fifty inches (7)
I believe the answer is:
pledges
'promises' is the definition.
(pledge is a kind of promise)
'plasma's first over fifty inches' is the wordplay.
'first' indicates taking the first letters.
'over' means one lot of letters go next to another.
'fifty' becomes 'l' (roman numeral).
'inches' becomes 'edges' ('edge' can be a synonym of 'inch').
The first letter of 'plasmas' is 'p'.
'p'+'l'+'edges'='PLEDGES'
(Other definitions for pledges that I've seen before include "Vows" , "Promises" , "Loyal toasts" .)