Build up credit union's leader with great coverage (6)
I believe the answer is:
accrue
'build up' is the definition.
(I've seen this in another clue)
'credit union's leader with great coverage' is the wordplay.
'credit' becomes 'cr' (abbreviation).
'leader' indicates taking the first letters.
'with' means one lot of letters go next to another.
'great' becomes 'ace' (ace can informally mean very good).
'coverage' is an insertion indicator.
The initial letter of 'unions' is 'u'.
'cr'+'u'='cru'
'cru' put inside 'ace' is 'ACCRUE'.
(Other definitions for accrue that I've seen before include "(Of money) be received in increasing amounts" , "Grow like interest on money" , "Accumulate or grow like interest" , "(Of money) build up in regular amounts" , "(Of money) build up by regular receipts" .)