Advanced English force in manoeuvres - they might be in tanks (9)
I believe the answer is:
angelfish
'they might be in tanks' is the definition.
'advanced english force in manoeuvres' is the wordplay.
'advanced' becomes 'a'.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'in manoeuvres' is an anagram indicator (I've seen this in other clues).
'english'+'f'='englishf'
'englishf' anagrammed gives 'ngelfish'.
'a'+'ngelfish'='ANGELFISH'
(Other definitions for angelfish that I've seen before include "Brightly-coloured swimmer" , "shark" .)