Tested guided missile power at first (7)
I believe the answer is:
Here is my best explanation:
'tested' is the definition.
Although both the answer and definition are past participle verbs, I don't understand how one could define the other.
'guided missile power at first' is the subsidiary indication.
'guided' becomes 'led' ('lead' can be a synonym of 'guide').
'missile' becomes 'sam' (SAM is an example).
'power' becomes 'p'.
'at first' means one lot of letters go next to another.
'led' after 'samp' is 'sampled'.
(Other definitions for sampled that I've seen before include "Tried", "Tasted" and "tried".)