Soldiers left part of church seeing deterioration (7)
I believe the answer is:
relapse
'deterioration' is the definition.
(thesaurus)
'soldiers left part of church' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'left' becomes 'l' (common abbreviation).
'part of church' becomes 'apse' ('apse' is part of 'church').
're'+'l'+'apse'='RELAPSE'
'seeing' acts as a link.
(Other definitions for relapse that I've seen before include "Fall back into former condition, being pleaser" , "New downturn in health" , "Fall back into old ways" , "Degenerate like pleaser" , "Become ill again" .)