Sufficient to follow on after second test (6)
I believe the answer is:
sample
'test' is the definition.
(I've seen this before)
'sufficient to follow on after second' is the wordplay.
I cannot quite understand how this works, but
'sufficient' could be 'ample' (both can mean enough) and 'ample' is present in the answer.
'second' could be 's' (common abbreviation) and 's' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sample that I've seen before include "Specimen, try out" , "Taste; specimen" , "Small part representing the whole" , "Drop of" , "Taste(r)" .)