Pertinence of soldiers on enclave being mobilised (9)
I believe the answer is:
relevance
'pertinence' is the definition.
(I've seen this before)
'soldiers on enclave being mobilised' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'on' is an insertion indicator (as in clothing 'on' a person).
'being mobilised' indicates anagramming the letters (I've seen this in other clues).
'enclave' is an anagram of 'elevanc'.
're' going around 'elevanc' is 'RELEVANCE'.
'of' acts as a link.
(Other definitions for relevance that I've seen before include "Point" , "shows importance to the matter in hand" , "Aptness" , "Bearing" , "Pertinence to the matter in hand" .)