Advanced English force in manoeuvres - they might be in tanks (9)
I believe the answer is:
Here is my best explanation:
'they might be in tanks' is the definition.
'advanced english force in manoeuvres' is the subsidiary indication.
'advanced' becomes 'a'.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'in manoeuvres' indicates anagramming the letters (I've seen this in other clues).
'englishf' with letters rearranged gives 'ngelfish'.
(Other definitions for angelfish that I've seen before include "Brightly-coloured swimmer", "Swimmer" and "shark".)